smartmatch: handle stack realloc

smartmatch: handle stack realloc

When smartmatch is matching a pattern against something, it was
failing to do appropriate PUTBACK and SPAGAIN's  before calling
matcher_matches_sv() (which pushes an arg an calls pp_match()).
If the stack was almost full, the extra push in matcher_matches_sv()
could cause a stack realloc, which would then be ignored when
pp_smartmatch() returned, setting PL_stack_sp to point to the old (freed)
stack.

Adding SPAGAIN ensures that PL_stack_sp points to the new stack, while
PUTBACK causes PL_stack_sp to no longer see the two args to pp_smartmatch,
so the PUSH in matcher_matches_sv() pushes the SV us9ng ones of two two
reclaimed slots, so the stack won't re-alloc anyway.

Thus by doing the "right thing" with both PUTBACK and SPAGAIN, we
doubly ensure that PL_stack_sp will always be right.