[RT #129069] Perl_yylex: Fix two use-after-free bugs

[RT #129069] Perl_yylex: Fix two use-after-free bugs

Perl_yylex maintains up to two pointers, `s` and `d`, into the parser
buffer at PL_bufptr. It can call skipspace(), which can potentially
grow (and realloc) its argument. This can leave the second pointer
pointing at the old buffer. Under most cases it isn't visible, because
the old buffer isn't reused or zeroed. However, under Valgrind or
libdislocator, this memory management error becomes visible.

This patch saves the location of the second pointer in two locations,
and restores it after the call to skipspace.