Do not call BCOPY and BZERO if size is zero

Do not call BCOPY and BZERO if size is zero

* dbg_mlc.c (GC_debug_strndup): Do not call BCOPY() if elements count
is zero.
* dbg_mlc.c (GC_debug_realloc): Likewise.
* finalize.c [!GC_TOGGLE_REFS_NOT_NEEDED] (ensure_toggleref_capacity):
Likewise.
* malloc.c [REDIRECT_MALLOC && !strndup] (strndup): Likewise.
* mallocx.c (GC_strndup): Likewise.
* misc.c [!GC_GET_HEAP_USAGE_NOT_NEEDED] (GC_get_prof_stats): Likewise.
* os_dep.c [SAVE_CALL_CHAIN && GC_HAVE_BUILTIN_BACKTRACE]
(GC_save_callers): Likewise.
* reclaim.c (GC_print_all_errors): Likewise.
* malloc.c (GC_free): Do not call BZERO() if elements count is zero.
* malloc.c [THREADS] (GC_free_inner): Likewise.
* reclaim.c (GC_print_all_errors): Likewise.
* misc.c [!GC_GET_HEAP_USAGE_NOT_NEEDED && THREADS]
(GC_get_prof_stats_unsafe): Do not call fill_prof_stats() and BCOPY()
if stats_sz is zero.