error: change AVERROR_EOF value

The current value is masking the POSIX error code EPIPE, which has a
different semantics.

This breaks API.
Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/error.h

@@ -37,7 +37,7 @@
 #define AVUNERROR(e) (e)
 #endif
 
-#define AVERROR_EOF         AVERROR(EPIPE)   ///< End of file
+#define AVERROR_EOF             (-MKTAG('E','O','F',' ')) ///< End of file
 
 #define AVERROR_PATCHWELCOME    (-MKTAG('P','A','W','E')) ///< Not yet implemented in Libav, patches welcome