make out of order read a fatal error

For png and jpg, out of order must always be a fatal error, since you
can't read from a specific y position.

This was a g_assert() before which meant that this test was missing from
production builds. It's now always a fatal error.

ChangeLog

@@ -5,6 +5,7 @@
 - fix memleak in VipsArray [Lovell]
 - fix memleak in webp load from buffer [Lovell]
 - fix memleak in png save to buffer [Lovell]
+- make out of order read in png and jpg a fatal error
 
 12/8/14 started 7.40.6
 - more doc fixes

libvips/foreign/jpeg2vips.c

@@ -916,7 +916,11 @@ read_jpeg_generate( VipsRegion *or,
 	/* And check that y_pos is correct. It should be, since we are inside
 	 * a vips_sequential().
 	 */
-	g_assert( r->top == jpeg->y_pos ); 
+	if( r->top != jpeg->y_pos ) {
+		vips_error( "VipsJpeg", 
+			_( "out of order read at line %d" ), jpeg->y_pos );
+		return( -1 );
+	}
 
 	/* Here for longjmp() from vips__new_error_exit().
 	 */

libvips/foreign/vipspng.c

@@ -483,7 +483,11 @@ png2vips_generate( VipsRegion *or,
 	/* And check that y_pos is correct. It should be, since we are inside
 	 * a vips_sequential().
 	 */
-	g_assert( r->top == read->y_pos ); 
+	if( r->top != read->y_pos ) {
+		vips_error( "vipspng", 
+			_( "out of order read at line %d" ), read->y_pos );
+		return( -1 );
+	}
 
 	for( y = 0; y < r->height; y++ ) {
 		png_bytep q = (png_bytep) VIPS_REGION_ADDR( or, 0, r->top + y );