fix error message for metadata get

if the type of the metadata item didn't match theh gtype you were
fetching to, the error message was wrong

ChangeLog

@@ -1,5 +1,6 @@
 27/9/16 started 8.4.2
 - small doc improvements
+- fix error message for metadata fetch type mismatch
 
 1/5/16 started 8.4
 - many more wepsave options [Felix Bünemann]

libvips/iofuncs/header.c

@@ -1225,7 +1225,7 @@ meta_get_value( const VipsImage *image,
 		vips_error( "VipsImage",
 			_( "field \"%s\" is of type %s, not %s" ),
 			field,
-			g_type_name( G_VALUE_TYPE( value_copy ) ),
+			g_type_name( G_VALUE_TYPE( &value ) ),
 			g_type_name( type ) );
 		g_value_unset( &value );