[patch, Fortran, committed] Fix a crash with a developer's option

[Thomas Koenig]

Committed as obvious, simple and risk-free after checking
that it still compiles.

It does not fix the PR in question, it it just something that
was noted in one of the comments.

Regards

	Thomas

Avoid crash when global symbol table is empty with -fdump-fortran-global.

This just avoids a crash with -fdump-fortran-global when the
global symbol table is empty.  This is strictly a developer's
option, no user impact.

gcc/fortran/ChangeLog:

2020-06-14  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR fortran/42122
	* dump-parse-tree.c (gfc_dump_global_symbols):  If the symroot is
	empty, just output "empty".
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