Fix use of `check_zero()` in `forceNetwork()` and `sankeyNetwork()`

[lewinfox]

Fixes #281

Calling check_zero() too early in the function resulted in a bug where 1-indexed input would not be detected if either:

• Links was not a data.frame
• Source or Target were missing

Moving check_zero() further down the functions and adding missing input checks for forceNetwork() fixes the issue.