Fix compiler warning about possible-NULL-dreference

* src/mpi.h (mpi_is_const, mpi_is_immutable): Do check arg before
deref-ing.  The are only used at places where the arg shall not be NULL.

This was designed as a general purpose macro and written in a
defensive way. However, if it a NULL would be passed to that macro
code run in the else branch will deref the arg anyway.

• Signed-off-by: Werner Koch <wk@gnupg.org>