Static: fixed potential undefined behavior in memcpy().

According to the C standard, pointer arguments passed to memcpy() calls shall
still have valid values.  NULL is considered as invalid.

Found with GCC Static Analyzer.

src/nxt_http_static.c

@@ -76,7 +76,7 @@ nxt_http_static_handler(nxt_task_t *task, nxt_http_request_t *r,
         nxt_str_set(&extension, ".html");
 
     } else {
-        nxt_str_null(&index);
+        nxt_str_set(&index, "");
         nxt_str_null(&extension);
     }