flang2 terminates with segfault when run directly without arguments

[dev-zero]

While there is much point in running flang2 manually without arguments, it should still not just segfault:

$ flang2
Segmentation fault (core dumped)

gdb backtrace:

(gdb) bt
#0  0x000000000057f344 in parse_arguments ()
#1  0x000000000050e2b9 in init ()
#2  0x000000000050da80 in main ()

[sscalpone]

% /proj/pgi/flang/x86_64/dev/bin/flang1 < /dev/null
Segmentation fault

[Renaud-K]

Created #724 to address this issue