src/expand.c: avoid assignment of incompatibple pointers

[trofi]

Upcoming gcc-14 added and enabled as an error by default the check for assignment of incompatible pointers. There mcpp build fails as:

expand.c: In function 'replace':
expand.c:713:21: error: assignment to 'char *' from incompatible pointer type 'LOCATION *' {aka 'struct location *'} [-Wincompatible-pointer-types]
  713 |         m_inf->args = m_inf->loc_args = NULL;       /* Default args */
      |                     ^

The mismatch happens because args and loc_args have incompatible types:

char *          args;
LOCATION *      loc_args;

The change splits pointer assignments in two.