Sporadic crashes with some applications in RHEL-5.4 or later when _MALLOC_CHECK_ environment variable is set

Issue

• Sporadic crashes with some applications in RHEL-5.4 or later when MALLOC_CHECK_ environment variable is set
• The crashes stop when the following chunk is reversed in glibc-expmalloc2.patch:

if(!mem) return;
-  (void)mutex_lock(&main_arena.mutex);
   p = mem2chunk_check(mem, NULL);
   if(!p) {
-    (void)mutex_unlock(&main_arena.mutex);