regcomp.c: Don't read uninitialized data

I keep forgetting that the OP of a regnode is not defined in Pass 1 of
the regex compiler.  This is likely the cause of inconsistent results in
lib/locale.t, as valgrind shows there to be a read of uninitialized
data before this patch, and the result is randomly tainting when there
shouldn't be, consistent with the test failures.