Fix some compiler warnings

[smcv]

• dir: Consistently initialize g_autofree variables

  All g_autofree and g_autoptr variables should be initialized, either
  with a value or NULL, and some gcc versions warn if they are not.

  This particular instance was harmless, because we initialized regexp
  to either NULL or a valid non-NULL value as the first statement of the
  function, so there was no way to return before regexp was initialized;
  but if we introduced an early-return such as a g_return_val_if_fail
  before the current first statement, then that early return would have
  caused uninitialized stack contents to be freed, likely resulting in a
  crash.

• run: Make it clearer that rest_argv_start is not used uninitialized

  rest_argv_start is initialized whenever rest_argc != 0, so the previous
  code was in fact safe; but this wasn't obvious to either a human reader
  or the compiler, and some gcc versions warn here.

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[pwithnall]

This LGTM, for what it’s worth

[smcv]

I'm a maintainer, @pwithnall is a frequent contributor, I'm going to call that good enough.