xcap_server: rever use of STR_VTOZ macro

- got it by picking a commit with another fix and this macro is not in
branch 5.3

src/modules/xcap_server/xcap_server.c

@@ -321,12 +321,13 @@ int xcaps_xpath_hack(str *buf, int type)
 	}
 
 	start = buf->s;
-	STR_VTOZ(buf->s[buf->len-1], c);
+	c = buf->s[buf->len-1];
+	buf->s[buf->len-1] = '\0';
 	while((start < buf->s + buf->len - 8) && (p = strstr(start, match))!=NULL) {
 		memcpy(p, repl, 7);
 		start = p + 7;
 	}
-	STR_ZTOV(buf->s[buf->len-1], c);
+	buf->s[buf->len-1] = c;
 	return 0;
 }