avformat/aviobuf: Fix end check in put_str16()

Fixes out of array read
Fixes: 03c406ec9530e594a074ce2979f8a1f0/asan_heap-oob_7dec26_4664_37c52495b2870a2eaac65f53958e76c1.flac

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 115fb6d0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/aviobuf.c

@@ -358,6 +358,8 @@ int avio_put_str16le(AVIOContext *s, const char *str)
 invalid:
         av_log(s, AV_LOG_ERROR, "Invaid UTF8 sequence in avio_put_str16le\n");
         err = AVERROR(EINVAL);
+        if (!*(q-1))
+            break;
     }
     avio_wl16(s, 0);
     if (err)