C-Allocators: Mention realloc behavior if size is zero

Signed-off-by: Daiki Ueno <dueno@redhat.com>

modules/ROOT/pages/programming-languages/C-Allocators.adoc

@@ -43,6 +43,20 @@ compiler may assume that a comparison between the old and new
 pointer will always return false, so it is impossible to detect
 movement this way.
 
+On a related note, `realloc` frees the memory area if the new size is
+zero. If the size unintentionally becomes zero, as a result of
+unsigned integer wrap-around for instance, the following idiom causes
+a double-free.
+
+[source,c]
+----
+new_size = size + x; /* 'x' is a very large value and the result wraps around to zero */
+new_ptr = realloc(ptr, new_size);
+if (!new_ptr) {
+	free(ptr);
+}
+----
+
 ==== Handling Memory Allocation Errors
 
 Recovering from out-of-memory errors is often difficult or even