error: ensure error codes are signed negative ints.

Based on a patch by Robert Nagy <ronag89@gmail.com>.

It makes a difference when the error code is immediately cast
into a larger integer, such as an int64_t.

libavutil/error.h

@@ -44,27 +44,29 @@
 #define AVUNERROR(e) (e)
 #endif
 
-#define AVERROR_BSF_NOT_FOUND      (-MKTAG(0xF8,'B','S','F')) ///< Bitstream filter not found
-#define AVERROR_BUG                (-MKTAG( 'B','U','G','!')) ///< Internal bug, also see AVERROR_BUG2
-#define AVERROR_DECODER_NOT_FOUND  (-MKTAG(0xF8,'D','E','C')) ///< Decoder not found
-#define AVERROR_DEMUXER_NOT_FOUND  (-MKTAG(0xF8,'D','E','M')) ///< Demuxer not found
-#define AVERROR_ENCODER_NOT_FOUND  (-MKTAG(0xF8,'E','N','C')) ///< Encoder not found
-#define AVERROR_EOF                (-MKTAG( 'E','O','F',' ')) ///< End of file
-#define AVERROR_EXIT               (-MKTAG( 'E','X','I','T')) ///< Immediate exit was requested; the called function should not be restarted
-#define AVERROR_FILTER_NOT_FOUND   (-MKTAG(0xF8,'F','I','L')) ///< Filter not found
-#define AVERROR_INVALIDDATA        (-MKTAG( 'I','N','D','A')) ///< Invalid data found when processing input
-#define AVERROR_MUXER_NOT_FOUND    (-MKTAG(0xF8,'M','U','X')) ///< Muxer not found
-#define AVERROR_OPTION_NOT_FOUND   (-MKTAG(0xF8,'O','P','T')) ///< Option not found
-#define AVERROR_PATCHWELCOME       (-MKTAG( 'P','A','W','E')) ///< Not yet implemented in FFmpeg, patches welcome
-#define AVERROR_PROTOCOL_NOT_FOUND (-MKTAG(0xF8,'P','R','O')) ///< Protocol not found
-#define AVERROR_STREAM_NOT_FOUND   (-MKTAG(0xF8,'S','T','R')) ///< Stream not found
+#define FFERRTAG(a, b, c, d) (-(int)MKTAG(a, b, c, d))
+
+#define AVERROR_BSF_NOT_FOUND      FFERRTAG(0xF8,'B','S','F') ///< Bitstream filter not found
+#define AVERROR_BUG                FFERRTAG( 'B','U','G','!') ///< Internal bug, also see AVERROR_BUG2
+#define AVERROR_DECODER_NOT_FOUND  FFERRTAG(0xF8,'D','E','C') ///< Decoder not found
+#define AVERROR_DEMUXER_NOT_FOUND  FFERRTAG(0xF8,'D','E','M') ///< Demuxer not found
+#define AVERROR_ENCODER_NOT_FOUND  FFERRTAG(0xF8,'E','N','C') ///< Encoder not found
+#define AVERROR_EOF                FFERRTAG( 'E','O','F',' ') ///< End of file
+#define AVERROR_EXIT               FFERRTAG( 'E','X','I','T') ///< Immediate exit was requested; the called function should not be restarted
+#define AVERROR_FILTER_NOT_FOUND   FFERRTAG(0xF8,'F','I','L') ///< Filter not found
+#define AVERROR_INVALIDDATA        FFERRTAG( 'I','N','D','A') ///< Invalid data found when processing input
+#define AVERROR_MUXER_NOT_FOUND    FFERRTAG(0xF8,'M','U','X') ///< Muxer not found
+#define AVERROR_OPTION_NOT_FOUND   FFERRTAG(0xF8,'O','P','T') ///< Option not found
+#define AVERROR_PATCHWELCOME       FFERRTAG( 'P','A','W','E') ///< Not yet implemented in FFmpeg, patches welcome
+#define AVERROR_PROTOCOL_NOT_FOUND FFERRTAG(0xF8,'P','R','O') ///< Protocol not found
+#define AVERROR_STREAM_NOT_FOUND   FFERRTAG(0xF8,'S','T','R') ///< Stream not found
 
 /**
  * This is semantically identical to AVERROR_BUG
  * it has been introduced in Libav after our AVERROR_BUG and with a modified value.
  */
-#define AVERROR_BUG2               (-MKTAG( 'B','U','G',' '))
-#define AVERROR_UNKNOWN            (-MKTAG( 'U','N','K','N')) ///< Unknown error, typically from an external library
+#define AVERROR_BUG2               FFERRTAG( 'B','U','G',' ')
+#define AVERROR_UNKNOWN            FFERRTAG( 'U','N','K','N') ///< Unknown error, typically from an external library
 
 /**
  * Put a description of the AVERROR code errnum in errbuf.