[patch, libfortran] Fix EXECUTE_COMMAND_LINE error return

[Thomas Koenig]

Hello world,

currently, EXECUTE_COMMAND_LINE aborts the program if the called program
is not found, if cmdstat is present, but cmdmsg isn't. This is a 6/7/8
regression.

This patch fixes the issue by simply removing the erronous call to
runtime_error.

Regression-tested.  OK for all affected branches?

Regards

	Thomas

2017-10-10  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR libfortran/82233
         * intrinsics/execute_command_line.c (execute_command_line):
         No call to runtime_error if cmdstat is present.

2017-10-10  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR libfortran/82233
         * gfortran.dg/execute_command_line_3.f90: New test.
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