lavu: add AVERROR_BUG error value

It should be used to mark codepath that can be reached only through
programming error.

libavutil/error.c

@@ -38,6 +38,7 @@ int av_strerror(int errnum, char *errbuf, size_t errbuf_size)
     case AVERROR_PATCHWELCOME:      errstr = "Not yet implemented in Libav, patches welcome"; break;
     case AVERROR_PROTOCOL_NOT_FOUND:errstr = "Protocol not found"                           ; break;
     case AVERROR_STREAM_NOT_FOUND:  errstr = "Stream not found"                             ; break;
+    case AVERROR_BUG:               errstr = "Bug detected, please report the issue"        ; break;
     }
 
     if (errstr) {

libavutil/error.h

@@ -57,6 +57,7 @@
 #define AVERROR_PATCHWELCOME       (-MKTAG( 'P','A','W','E')) ///< Not yet implemented in Libav, 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 AVERROR_BUG                (-MKTAG( 'B','U','G',' ')) ///< Bug detected, please report the issue
 
 /**
  * Put a description of the AVERROR code errnum in errbuf.