Silence compiler warning

The Appveyor build generates this otherwise:

In function 'Jim_WhileCoreCommand',
    inlined from 'Jim_WhileCoreCommand' at jim.c:12096:12:
jim.c:12109:12: warning: 'boolean' may be used uninitialized [-Wmaybe-uninitialized]
12109 |         if (!boolean)
      |            ^
jim.c: In function 'Jim_WhileCoreCommand':
jim.c:12105:13: note: 'boolean' was declared here
12105 |         int boolean, retval;
      |             ^~~~~~~