Fix show-model option

[zoziha]

To close #615 .

Init the f_source%link_libraries array length as 0.

[St-Maxwell]

It's OK.

[milancurcic]

Looks good, the issue from #615 goes away for me with this patch. Thank you!

[zoziha]

This is a relatively small correction, mainly to initialize the length of the allocatable array f_source%link_libraries to prevent access segfaults.
If possible, it seems that the PR can pass ~

[LKedward]

Thanks for this @zoziha and many apologies for the delay in reviewing. I needed to double-check why this wasn't breaking the link_executable example package test case, but this all looks fine to me.

The actual link_libraries are populated in the add_executable_sources routine which applies any executable-specific information from the manifest (such as link_libraries). Your changes don't break this because of (re)allocation-on-assignment.