fix a couple of issues with huge profiles

The base64 encode limit was 1MB, but some huge profiles can get over
that, so it's now 10MB.

transform_save_string_blob() was incorrectly setting a refstring not a
blob as an error fallback, potentially leaving a dangling pointer.

Thanks Jaume!

See https://github.com/jcupitt/libvips/issues/666

ChangeLog

@@ -1,5 +1,7 @@
 19/5/17 started 8.5.6
 - tiff read with start page > 0 could break edge tiles or strips
+- raise b64 limit to allow for huge profiles (thanks jaume)
+- fix error return in blob save (thanks jaume)
 
 23/4/17 started 8.5.5
 - doc polishing

libvips/iofuncs/base64.c

@@ -65,7 +65,7 @@ Modified on:
 	- move to vips_ namespace
 
 31/5/15
-	- oops siged/unsignned mess-up meant we were not padding correctly
+	- oops siged/unsigned mess-up meant we were not padding correctly
 
  */
 
@@ -237,7 +237,9 @@ vips__b64_decode( const char *buffer, size_t *data_length )
 	int nbits;
 	int i;
 
-	if( output_data_length > 1024 * 1024 ) {
+	/* A large ICC profile can be a couple of MB, so 10 should be plenty.
+	 */
+	if( output_data_length > 10 * 1024 * 1024 ) {
 		/* We shouldn't really be used for large amounts of data, plus
 		 * we are using an int for offset.
 		 */

libvips/iofuncs/type.c

@@ -692,7 +692,7 @@ transform_save_string_blob( const GValue *src_value, GValue *dest_value )
 		/* No error return from transform, but we should set it to
 		 * something.
 		 */
-		vips_value_set_save_string( dest_value, "" ); 
+		vips_value_set_blob( dest_value, NULL, NULL, 0 ); 
 }
 
 GType