Avoid undefined behaviour when copying part of structure

* src/dispnew.c (copy_row_except_pointers): Don't use address of
subobject as starting point.

(cherry picked from commit 6943786b5c)

src/dispnew.c

@@ -1034,7 +1034,7 @@ copy_row_except_pointers (struct glyph_row *to, struct glyph_row *from)
 {
   enum { off = offsetof (struct glyph_row, x) };
 
-  memcpy (&to->x, &from->x, sizeof *to - off);
+  memcpy ((char *) to + off, (char *) from + off, sizeof *to - off);
 }