regcomp.c: RT#77414. Initialize flag

regcomp.c: RT#77414. Initialize flag

As indicated in the comments, this flag needs to be initialized to
1 or the optimizer loses the fact that something could match a
character that isn't in utf8 and whose bitmap bit isn't set.  This
happens, for example, with Unicode properties.

Thus this fixes #77414.  That ticket had been closed recently because
it went away due to another patch that caused the optimizer to be
bypassed in the cases tested for.  But when that patch was reverted,
and cleaned-up, this bug came back.  Now, I believe I have found the
root cause.