regcomp.c: Fix \N{} multi-char fold buffer boundary bug

regcomp.c: Fix \N{} multi-char fold buffer boundary bug

An earlier commit in this topic branch fixed the bug (for non-\N{})
cases where a multi-character fold could try to span two EXACTFish
nodes, where they are split because the first one would otherwise
contain too long a string.

This commit extends that fix to include characters entered via \N{...}.
It does this by causing \N handling to be split, so that if the \N
resolves to a single code point, it goes through the normal processing,
so that it no longer bypasses the code that was added in the earlier
commit.