remove `unaligned store` UBsan warnings

This patch addresses several warnings from `make test` when
zlib-ng was configured -with-fuzzers -with-sanitizers:

zlib-ng/trees.c:798:5: runtime error: store to misaligned address 0x63100125c801 for type 'uint16_t' (aka 'unsigned short'), which requires 2 byte alignment
0x63100125c801: note: pointer points here
 00 80 76  01 8b 08 00 00 00 00 00  00 03 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00
              ^
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior zlib-ng/trees.c:798:5 in
zlib-ng/trees.c:799:5: runtime error: store to misaligned address 0x63100125c803 for type 'uint16_t' (aka 'unsigned short'), which requires 2 byte alignment
0x63100125c803: note: pointer points here
 76  01 f5 08 00 00 00 00 00  00 03 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00
              ^

Instead of using `*(uint16_t*) foo = bar` to write a uint16_t, call
__builtin_memcpy which will be safe in case of memory page boundaries.

Without the patch:

 Performance counter stats for './minigzip -9 llvm.tar':

      13173.840115      task-clock (msec)         #    1.000 CPUs utilized
                27      context-switches          #    0.002 K/sec
                 0      cpu-migrations            #    0.000 K/sec
               129      page-faults               #    0.010 K/sec
    57,801,072,298      cycles                    #    4.388 GHz
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
    75,270,723,557      instructions              #    1.30  insns per cycle
    17,797,368,302      branches                  # 1350.963 M/sec
       196,795,107      branch-misses             #    1.11% of all branches

      13.177897531 seconds time elapsed

45408 -rw-rw-r-- 1 spop spop 46493896 Dec 11 14:45 llvm.tar.gz

With remove-unaligned-stores patch:

      13184.736536      task-clock (msec)         #    1.000 CPUs utilized
                44      context-switches          #    0.003 K/sec
                 1      cpu-migrations            #    0.000 K/sec
               129      page-faults               #    0.010 K/sec
    57,882,724,316      cycles                    #    4.390 GHz
   <not supported>      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend
    75,235,920,853      instructions              #    1.30  insns per cycle
    17,826,873,999      branches                  # 1352.084 M/sec
       196,050,096      branch-misses             #    1.10% of all branches

      13.185868238 seconds time elapsed

45408 -rw-rw-r-- 1 spop spop 46493896 Dec 11 14:46 llvm.tar.gz

1 files changed, 7 insertions, 19 deletions

diff --git a/deflate.h b/deflate.h
index 02d8f5a..6cba8dd 100644
--- a/deflate.h
+++ b/deflate.h
@@ -13,6 +13,7 @@
 /* @(#) $Id$ */
 
 #include "zutil.h"
+#include "gzendian.h"
 
 /* define NO_GZIP when compiling if you want to disable gzip header and
    trailer creation by deflate().  NO_GZIP would be used to avoid linking in
@@ -302,26 +303,13 @@ typedef enum {
  * Output a short LSB first on the stream.
  * IN assertion: there is enough room in pendingBuf.
  */
-#ifdef UNALIGNED_OK
-/* Compared to the else-clause's implementation, there are few advantages:
- *  - s->pending is loaded only once (else-clause's implementation needs to
- *    load s->pending twice due to the alias between s->pending and
- *    s->pending_buf[].
- *  - no instructions for extracting bytes from short.
- *  - needs less registers
- *  - stores to adjacent bytes are merged into a single store, albeit at the
- *    cost of penalty of potentially unaligned access. 
- */
-#define put_short(s, w) { \
-    *(uint16_t*)(&s->pending_buf[s->pending]) = (w) ; \
-    s->pending += 2; \
-}
-#else
-#define put_short(s, w) { \
-    put_byte(s, (unsigned char)((w) & 0xff)); \
-    put_byte(s, (unsigned char)((uint16_t)(w) >> 8)); \
-}
+static inline void put_short(deflate_state *s, uint16_t w) {
+#if BYTE_ORDER == BIG_ENDIAN
+  w = ZSWAP16(w);
 #endif
+  MEMCPY(&(s->pending_buf[s->pending]), &w, sizeof(uint16_t));
+  s->pending += 2;
+}
 
 #define MIN_LOOKAHEAD (MAX_MATCH+MIN_MATCH+1)
 /* Minimum amount of lookahead, except at the end of the input file.