[lldb/Commands] Fix heap-use-after-free error in CommandObjectProcess

This patch should fix the use-after-free error that was brought up by
the LLDB ASAN Green Dragon bot.

This is caused because the `StringRef` object was acquired too early
before being use and by the underlying memory was modified which caused
it to point to null memory.

Fetching back the string reference close to its usage location should
fix the issue.

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>