_AS_PATH_WALK: Use AS_IF for IF-NOT-FOUND argument.

_AS_PATH_WALK: Use AS_IF for IF-NOT-FOUND argument.

The construct _AS_PATH_WALK was using to conditionally execute its
IF-NOT-FOUND argument, was a little too fragile: relatively natural
variations in usage, such as putting the final `])` on a line by
itself, could cause shell syntax errors.  Use AS_IF instead.

* lib/m4sugar/m4sh.m4: Use AS_IF to execute IF-NOT-FOUND conditionally.