S_regmatch(): combine CURLY_B_min/_known states

S_regmatch(): combine CURLY_B_min/_known states

There are currently two similar backtracking states for simple
non-greedy pattern repeats:

    CURLY_B_min
    CURLY_B_min_known

the latter is a variant of the former for when the character which must
follow the repeat is known, e.g.  /(...)*?X.../, which allows quick
skipping to the next viable position.

The code for the two cases:

    case CURLY_B_min_fail:
    case CURLY_B_min_known_fail:

share a lot of similarities. This commit merges the two states into a
single CURLY_B_min state, with an associated single CURLY_B_min_fail
fail state.

That one code block can handle both types, with a single

    if (ST.c1 == CHRTEST_VOID) ...

test to choose between the two variant parts of the code.

This makes the code smaller and more maintainable, at the cost of one
extra test per backtrack.