array: Don't use realloc() with zero size in array_compress()

The behavior of realloc(3) with zero size was apparently implementation
defined.  While glibc documents the behavior as equivalent to free(3),
that might not apply to other C libraries.  With C17, this behavior has
been deprecated, and with C23, the behavior is now undefined.  It's also
why valgrind warns about this use.

Hence, when array_compress() would call realloc() with a zero size, we
now call free() explicitly and set the pointer to NULL.

Signed-off-by: Thomas Egerer <thomas.egerer@secunet.com>

src/libstrongswan/collections/array.c

@@ -197,7 +197,17 @@ void array_compress(array_t *array)
 		}
 		if (tail)
 		{
-			array->data = realloc(array->data, get_size(array, array->count));
+			size_t size = get_size(array, array->count);
+
+			if (size)
+			{
+				array->data = realloc(array->data, size);
+			}
+			else
+			{
+				free(array->data);
+				array->data = NULL;
+			}
 			array->tail = 0;
 		}
 	}