Fix zrealloc to behave similarly to je_realloc when size is 0

According to C11, the behavior of realloc with size 0 is now deprecated.
it can either behave as free(ptr) and return NULL, or return a valid pointer.
but in zmalloc it can lead to zmalloc_oom_handler and panic.
and that can affect modules that use it.

It looks like both glibc allocator and jemalloc behave like so:
  realloc(malloc(32),0) returns NULL
  realloc(NULL,0) returns a valid pointer

This commit changes zmalloc to behave the same

src/zmalloc.c

@@ -147,6 +147,10 @@ void *zrealloc(void *ptr, size_t size) {
     size_t oldsize;
     void *newptr;
 
+    if (size == 0 && ptr!=NULL) {
+        zfree(ptr);
+        return NULL;
+    }
     if (ptr == NULL) return zmalloc(size);
 #ifdef HAVE_MALLOC_SIZE
     oldsize = zmalloc_size(ptr);