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: NMichael Niedermayer <michael@niedermayer.cc>

libavformat/aviobuf.c

@@ -361,6 +361,8 @@ static inline int put_str16(AVIOContext *s, const char *str, const int be)
 invalid:
         av_log(s, AV_LOG_ERROR, "Invaid UTF8 sequence in avio_put_str16%s\n", be ? "be" : "le");
         err = AVERROR(EINVAL);
+        if (!*(q-1))
+            break;
     }
     if (be)
         avio_wb16(s, 0);