Build warnings in `regexec.c`

[leonerd]

Latest blead is generating warnings when building:

In file included from perl.h:4229,
                 from regexec.c:76:
regexec.c: In function ‘S_isWB’:
regexec.c:6276:28: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
 6276 |             assert(index+1 < C_ARRAY_LENGTH(WB_dfa_table));
      |                            ^

[leonerd]

git blame suggests a1fcbed as the source of that line:

commit a1fcbed
Author: Lukas Mai lukasmai.403@gmail.com
Date: Tue Sep 23 04:23:07 2025 +0200

regexec: assert we stay in bounds when matching \b{wb}

Coverity (CID 550207) complains that the loop condition only ensures
'index' stays in bounds of WB_dfa_table, not 'index+1'. I don't think
the data in the table lets us go out of bounds here, but it doesn't hurt
to assert() that assumption.

@mauke - want to take a look?