mesa/bindless: fix missing image _Layer initialization

Some later code relies on _Layer to set first/last_layer. Make sure it's
always initialized.

Detected by valgrind's conditional jump/move with uninit value logic.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/mesa/main/texturebindless.c

@@ -327,6 +327,7 @@ get_image_handle(struct gl_context *ctx, struct gl_texture_object *texObj,
    } else {
       imgObj.Layered = GL_FALSE;
       imgObj.Layer = 0;
+      imgObj._Layer = 0;
    }
 
    /* Request a new image handle from the driver. */