newXS_len_flags() shouldn't change the line number on PL_curcop when warning.

newXS_len_flags() shouldn't change the line number on PL_curcop when warning.

This can actually generate incorrect line numbers in runtime warnings, when
XSUBs are redefined from calls made from BEGIN blocks, and the line number
from the opening brace of the begin block is mashed with the filename of the
current line. For compiletime warnings, PL_curcop == &PL_compiling, so the
line numbers will be correct whether taken from PL_compiling or PL_parser.

This code dates back to perl-5.000, when it was added to newXS(). It appears
to be a copy of code present in newSUB() since alpha 2.