RT 64804: tainting with index() of a constant

RT 64804: tainting with index() of a constant

Bug: http://rt.perl.org/rt3/Public/Bug/Display.html?id=64804
Bug-Debian: http://bugs.debian.org/291450
Origin: upstream, http://perl5.git.perl.org/perl.git/commit/3b36395d31cf0a2f3a017505cd0ea857a7acb5d1

At compile time, ck_index with a tainted constant set PL_tainted,
which remained on during the rest of compilation, tainting all other
constants.

Fix this by saving and restoring PL_tainted across the call to
fbm_compile, which is what sets PL_tainted.