Fix bug #18528 with crashes at startup during frameset restoration.

Fix bug #18528 with crashes at startup during frameset restoration.

 src/w32term.c (w32_read_socket): Don't use frame dimensions for
 resizing if GetClientRect returned an empty (0, 0, 0, 0)
 rectangle.  Check the return value of GetClientRect, and don't use
 the results if it didn't succeed.
 src/dispnew.c (change_frame_size_1): Recompute the frame dimensions
 in columns and lines after correcting the pixel dimensions in
 check_frame_size.
 (adjust_decode_mode_spec_buffer): Add assertion to avoid passing
 negative values to xrealloc.