Dear Fortranners,

compiling with -fsanitize=undefined shows that we did mishandle the
case where a missing optional argument is passed to another procedure.

Besides the example given in the PR, the existing testcase
fortran.dg/missing_optional_dummy_6a.f90 fails with:

gcc/testsuite/gfortran.dg/missing_optional_dummy_6a.f90:21:29: runtime error: load of null pointer of type 'integer(kind=4)'
gcc/testsuite/gfortran.dg/missing_optional_dummy_6a.f90:22:30: runtime error: load of null pointer of type 'integer(kind=4)'
gcc/testsuite/gfortran.dg/missing_optional_dummy_6a.f90:27:29: runtime error: load of null pointer of type 'integer(kind=4)'

The least invasive change - already pointed out by the reporter - is
to check the presence of the argument before dereferencing the data
pointer after the offset calculation.  This requires adjusting the
checking pattern for gfortran.dg/missing_optional_dummy_6a.f90.

Regtesting reminded me that procedures with bind(c) attribute are doing
their own stuff, which is why they need to be excluded here, otherwise
testcase bind-c-contiguous-4.f90 would regress on the expected output.

I've created a testcase that uses this PR's input as well as the lesson
learned from studying the bind(c) testcase and placed this in the asan
subdirectory.

There is a potential alternative solution which I did not pursue, as I
think it is more invasive, but also that I didn't succeed to implement:
A non-present dummy array argument should not need to get its descriptor
set up.  Pursuing this is probably not the right thing to do during the
current stage of development and could be implemented later.  If somebody
believes this is important, feel free to open a PR for this.

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald