Check for missing thread count argument for -s/--threads

[charliesheh]

Summary
Adds validation for the -s / --threads command line option to ensure a thread count argument is provided before accessing argv[i].

Previously, invoking:
openmc -s
could result in an out-of-range access and exception rather than returning a clean invalid argument error.

Changes Made

• Added argument existence check before reading thread count value
• Returns OPENMC_E_INVALID_ARGUMENT with a descriptive error message when no thread count is specified

Issue
Fixes #3353

[paulromano]

Thanks @charliesheh!