utils: Fix out-of-bounds write

We are adding terminating NUL into res[len]. To avoid the invalid write, let's allocate len + 1.

Fixes: ebassi/graphene#168

src/mutest-utils.c

@@ -367,7 +367,7 @@ string_split (const char *str,
       indent[indent_len] = '\0';
     }
 
-  char *res = malloc (sizeof (char) * len);
+  char *res = malloc (sizeof (char) * (len + 1));
   if (mutest_unlikely (res == NULL))
     mutest_oom_abort ();