input_stream: size==-1 means unknown size

Define the special value "-1" as "unknown size".  Previously, there
was no indicator for streams with unknown size, which might confuse
some decoders.

src/decoder/aac_plugin.c

@@ -186,7 +186,7 @@ static void aac_parse_header(AacBuffer * b, float *length)
 	if (length)
 		*length = -1;
 
-	fileread = b->inStream->size;
+	fileread = b->inStream->size >= 0 ? b->inStream->size : 0;
 
 	fillAacBuffer(b);