Fix coding-related core dumps with gcc -ftrapv.

Fix coding-related core dumps with gcc -ftrapv.

The code was computing A - B, where A and B are pointers, and B is
random garbage.  This can lead to core dumps on platforms that
have special pointer registers, and it also leads to core dumps on
x86-64 when compiled with gcc -ftrapv.  The fix is to compute
A - B only when B is initialized properly.
* coding.c (coding_set_source, coding_set_destination): Return void.
(coding_change_source, coding_change_destinations): New functions,
with the old behaviors of coding_set_source and coding_set_destination.
All callers that need an offset changed to use these new functions.