intel: fix an assertion failure. fix bug #15575

src/mesa/drivers/dri/intel/intel_tex_image.c

 	 postConvWidth = 32 / texelBytes;
 	 texImage->RowStride = postConvWidth;
       }
-      
-      assert(texImage->RowStride == postConvWidth);
+
+      if (!intelImage->mt) {      
+	  assert(texImage->RowStride == postConvWidth);
+      }
    }
 
    /* Release the reference to a potentially orphaned buffer.