detect read errors in sniff correctly

we has a signed/unsigned mixup :(

ChangeLog

@@ -3,6 +3,7 @@
 - better gifload behaviour for DISPOSAL_UNSPECIFIED [DarthSim]
 - ban ppm max_value < 0 
 - add fuzz corpus to dist
+- detect read errors correctly in source_sniff
 
 20/6/19 started 8.9.1
 - don't use the new source loaders for new_from_file or new_from_buffer, it

libvips/iofuncs/source.c

@@ -1153,7 +1153,7 @@ vips_source_length( VipsSource *source )
 }
 
 /**
- * vips_source_peek: 
+ * vips_source_sniff_at_most: 
  * @source: peek this source
  * @data: return a pointer to the bytes read here
  * @length: max number of bytes to read
@@ -1219,13 +1219,13 @@ unsigned char *
 vips_source_sniff( VipsSource *source, size_t length )
 {
 	unsigned char *data;
-	size_t bytes_read;
+	gint64 bytes_read;
 
 	if( vips_source_test_features( source ) )
 		return( NULL );
 
 	bytes_read = vips_source_sniff_at_most( source, &data, length );
-	if( bytes_read < length )
+	if( bytes_read < (gint64) length )
 		return( NULL );
 
 	return( data );

libvips/iofuncs/util.c

@@ -1036,6 +1036,10 @@ vips__seek_no_error( int fd, gint64 pos, int whence )
 #ifdef OS_WIN32
 	new_pos = _lseeki64( fd, pos, whence );
 #else /*!OS_WIN32*/
+	/* On error, eg. opening a directory and seeking to the end, lseek() 
+	 * on linux seems to return 9223372036854775807 ((1 << 63) - 1)
+	 * rather than (off_t) -1 for reasons I don't understand. 
+	 */
 	new_pos = lseek( fd, pos, whence );
 #endif /*OS_WIN32*/