Fix GCC -Wstringop-truncation warning

The code in testcase memberin_extend_c would end up without a
terminating nul if passed a 50 byte string, and then make_upper()
would run off the end of the buffer.

Fix by using strncat() (which always nul terminates the result, and
also doesn't zero-fill the tail of the buffer if the result is
shorter).

Examples/test-suite/memberin_extend_c.i

@@ -32,7 +32,8 @@ char *Person_name_get(Person *p) {
 }
 
 void Person_name_set(Person *p, char *val) {
-  strncpy(p->name,val,50);
+  p->name[0] = '\0';
+  strncat(p->name, val, sizeof(p->name) - 1);
   make_upper(p->name);
 }
 %}