glib: Fix pointer signedness

Author: sdroege

glib/src/gstring.rs

@@ -151,7 +151,7 @@ impl GStr {
         if ffi::g_utf8_validate(ptr as *const _, -1, &mut end_ptr) != ffi::GFALSE {
             Some(Self::from_utf8_with_nul_unchecked(slice::from_raw_parts(
                 ptr as *const u8,
-                end_ptr.offset_from(ptr) as usize + 1,
+                end_ptr.offset_from(ptr as *const u8) as usize + 1,
             )))
         } else {
             None