add simple string overflow test

test/simple-memory-corruption/string_overflow.c

@@ -0,0 +1,19 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <malloc.h>
+
+__attribute__((optimize(0)))
+int main(void) {
+    char *p = malloc(16);
+    if (!p) {
+        return 1;
+    }
+
+    size_t size = malloc_usable_size(p);
+    memset(p, 'a', size);
+    printf("overflow by %zu bytes\n", strlen(p) - size);
+
+    return 0;
+}