d511080814 introduced a regression where
running imhex with arguments (i.e. `imhex blob1.bin blob2.bin`) will
segfault due to a null pointer dereference.
### Implementation description
This patch updates `getSubCommands` to follow the same control flow as
`getFeatures`, where if the function returns a null pointer, it will
return gracefully rather than crash.
cc97c0e525