child_process: Add nullptr checks after allocations

Checklist

• tests and code linting passes
• a test and/or benchmark is included
• the commit message follows commit guidelines

Affected core subsystem(s)

child_process

Description of change

• Add checks against null pointers after allocations made when spawning child processes for arguments, environment variables, etc.
• Make sure that the out-of-range tests for setting the uid/gid work when uid_t/gid_t are unsigned types (e.g. Linux) and the supplied value is negative, to the degree to which that is possible.