regcomp.c: Make sure no future ambiguity on new alpha assertions

regcomp.c: Make sure no future ambiguity on new alpha assertions

This change adds a colon when checking if a string is an alpha
assertion.  This is defensive coding against an unlikely future change
wherein some new (*... construct is added that has the same initial
sequence as one of these assertions.  The colon acts as a terminator and
would automatically prevent any ambiguity.  One of these had the colon,
and hence was inconsistent with the rest.

Spotted by ilmari.