Fix minor problems found by static checking.

Fix minor problems found by static checking.

* buffer.c (Fdelete_all_overlays): Return nil.
* doc.c (Fsubstitute_command_keys):
* regex.c (WEAK_ALIAS):
* xdisp.c (redisplay_internal):
Move initialization down, to pacify GCC 4.7.1 -Wjump-misses-init.