[patch, fortran] Fix empty substrings when simplifying

[Thomas Koenig]

Hell world,

the attached patch is fairly self-explanatory, it fixes an out-of-memory
condition.

Regression-tested. OK for trunk?

Regards

	Thomas

2018-03-24  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/70068
         * expr.c (find_substring_ref): Change types of start, end
         and length variables to gfc_charlen_t. Set length to zero
         for empty substring.

2018-03-24  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/70068
         * gfortran.dg/substr_7.f90: New test.
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