gh-150717: Avoid mark-array allocation for groupless regex patterns (GH-150719)

gh-150717: Avoid mark-array allocation for groupless regex patterns (GH-150719)

state_init() always did PyMem_New(state->mark, groups*2), which for a
pattern with no capturing groups is PyMem_Malloc(0) -- a real allocation
(plus matching free) on every match/search/fullmatch call, for an array
that is never read: groupless patterns emit no MARK opcodes and group 0's
span is taken from state->start/ptr.

Guard the allocation with `if (pattern->groups)`. state->mark stays NULL
(set by the preceding memset), and both the error path and state_fini
already PyMem_Free(NULL) safely.