Produce the watchlog warning log in a way that is safer from a signal handler. Fix a memory leak in the backtrace generation function.

src/zmalloc.h

@@ -75,6 +75,7 @@ size_t zmalloc_used_memory(void);
 void zmalloc_enable_thread_safeness(void);
 float zmalloc_get_fragmentation_ratio(void);
 size_t zmalloc_get_rss(void);
+void zlibc_free(void *ptr);
 
 #ifndef HAVE_MALLOC_SIZE
 size_t zmalloc_size(void *ptr);