Fix bug.

m-string.h

@@ -280,8 +280,9 @@ string_cat(string_t v, const string_t v2)
   if (M_LIKELY (size > 0)) {
     stringi_fit2size(v, v->size + size + 1);
     assert (v2->ptr != NULL);
-    memcpy(&v->ptr[v->size], v2->ptr, size + 1);
+    memcpy(&v->ptr[v->size], v2->ptr, size);
     v->size += size;
+    v->ptr[v->size] = 0;
   }
   STRING_CONTRACT (v);
 }

tests/test-mstring.c

@@ -203,6 +203,9 @@ int main(void)
   assert(string_equal_str_p (s1, "QWERTY"));
   assert(string_equal_str_p (s2, "AZERTY"));
 
+  string_cat(s1, s1);
+  assert (string_equal_str_p(s1, "QWERTYQWERTY"));
+
   string_clear (s2);
   string_clear (s1);
   exit(0);