very cheap double-free checks in malloc

src/malloc/malloc.c

@@ -393,6 +393,8 @@ void *realloc(void *p, size_t n)
 		char *base = (char *)self - extra;
 		size_t oldlen = n0 + extra;
 		size_t newlen = n + extra;
+		/* Crash on realloc of freed chunk */
+		if ((uintptr_t)base < mal.brk) *(char *)0=0;
 		if (newlen < PAGE_SIZE && (new = malloc(n))) {
 			memcpy(new, p, n-OVERHEAD);
 			free(p);
@@ -454,6 +456,8 @@ void free(void *p)
 		size_t extra = self->data[-1];
 		char *base = (char *)self - extra;
 		size_t len = CHUNK_SIZE(self) + extra;
+		/* Crash on double free */
+		if ((uintptr_t)base < mal.brk) *(char *)0=0;
 		__munmap(base, len);
 		return;
 	}