fix failure of malloc to set errno on heap (brk) exhaustion

I wrongly assumed the brk syscall would set errno, but on failure it
returns the old value of the brk rather than an error code.

src/malloc/malloc.c

@@ -177,6 +177,7 @@ static struct chunk *expand_heap(size_t n)
 	return w;
 fail:
 	unlock(mal.brk_lock);
+	errno = ENOMEM;
 	return 0;
 }