[Patch, fortran] Fix PR 51338 - ICE with front-end optimization and assumed character lengths

[Thomas Koenig]

Hello world,

the attached patch fixes PR 51338.  I chose a different approach
from what Bud did in the PR, because I rather wanted to fix the
specific code path when two NULL expressions were handled to
gfc_dep_compare_expr where determining the right answer is
straightforward rather than possibly introducing a (future?) logic
error.

Regression-tested. OK for trunk?

	Thomas

2011-11-29  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/51338
         * dependency.c (are_identical_variables):  Handle case where
         end fields of substring references are NULL.

2011-11-29  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/51338
         * gfortran.dg/assumed_charlen_substring_1.f90:  New test.
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