Fixed access violation in a type-bound procedure of open_hashmap_type

[degawa]

Based on issue #704, I added a presence check for the optional argument exists in get_other_open_data to fix the access violation due to assignment to an optional variable that was not presented.

The tasks done are summarized as follows:

• Fixed stdlib_hashmap_open.f90.
• Built using gfortran 11.2 bundled with quickstart Fortran on Windows 10 with cmake 3.20.3.
• Executed cmake --build build --target test to run the tests and confirmed all tests passed.
• Executed examples below that were using the open_hashmap_type and confirmed those finished successfully.
  • example_hashmaps_entries.f90
  • example_hashmaps_loading.f90
  • example_hashmaps_rehash.f90
  • example_hashmaps_remove.f90
  • example_hashmaps_set_other_data.f90

closes #704

[jvdp1]

Thank you @degawa for this fix.