[minor] Fix UBSAN warning in lib/http1.c
```
/lib/http1.c:887:27: runtime error: null pointer passed as argument 2, which is declared to never be null
/usr/include/string.h:43:28: note: nonnull attribute specified here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /lib/http1.c:887:27
```
@@ -884,7 +884,7 @@ void finalostream_send(h2o_ostream_t *_self, h2o_req_t *req, h2o_iovec_t *inbufs
if (bufs[bufcnt].len != 0)
++bufcnt;
}
- memcpy(bufs + bufcnt, inbufs, sizeof(h2o_iovec_t) * inbufcnt);
+ h2o_memcpy(bufs + bufcnt, inbufs, sizeof(h2o_iovec_t) * inbufcnt);
bufcnt += inbufcnt;
if (self->chunked_buf != NULL && chunked_suffix.len != 0)
bufs[bufcnt++] = chunked_suffix;
