rt 122283 - do not recurse into GOSUB/GOSTART when not SCF_DO_SUBSTR

See also comments in patch. A complex regex "grammar" like that in
RT 122283 causes perl to take literally forever, and exhaust all
memory during the pattern optimization phase.

Unfortunately I could not track down exacty why this occured, but
it was very clear that the excessive recursion was unnecessary and
excessive. By simply eliminating the unncessary recursion performance
goes back to being acceptable.

I have not thought of a good way to test this change, so this patch
does not include any tests. Perhaps we can test it using alarm, but
I will follow up on that later.
restore color to debug diagnostics
add an assert that the length arg for UTF8f is non-negative

If we dont we will just hit a different more confusing assert
later. In production builds we zero elen so the args is assumed
empty.
regcomp.c: fix fencepost error in vFAIL at end of pattern

If RExC_parse was after RExC_end we passed a negative length
into the UTF8f macro/format, which then triggered asserts.

As part of this we add a new diagnostic to show when a
(?(condition)...) construct has gone unterminated, and tests for
the unterminated case, thus testing that vFAIL works properly
at the end of the string at the same time.