regexec.c: Fix logic error

regexec.c: Fix logic error

The function S_find_next_masked() could return a pointer to something
that wasn't wanted, returning prematurely due to a logic error I made.
This erroneous code is in 5.28.0, but I couldn't figure out any actual
bugs this caused, due to the circumstances it is called under.

The bug is I should have used 'xor' instead of complement and 'and'.
Thus trying to find 0x2f, with a mask of all F's also found 2e.