avformat/aviobuf: Fix signed integer overflow in avio_seek()

Signed integer overflow is undefined behavior.
Detected with clang and -fsanitize=signed-integer-overflow
Signed-off-by: Vitaly Buka <vitalybuka@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/aviobuf.c

@@ -259,6 +259,8 @@ int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
         offset1 = pos + (s->buf_ptr - s->buffer);
         if (offset == 0)
             return offset1;
+        if (offset > INT64_MAX - offset1)
+            return AVERROR(EINVAL);
         offset += offset1;
     }
     if (offset < 0)