Add check for invalid/incorrect mapunit in mksoiltexMod

[samsrabin]

See below. In that discussion, @billsacks noted:

This seems like a very good idea, but I haven't done enough analysis of this myself to know whether there are cases where we might expect the result to be zero after mapping. If you determine, through code analysis and/or experimentation, that this kind of check works, then I'd support having it in place!

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Discussed in #2746

^{Originally posted by samsrabin September 9, 2024}
Huge thanks to @billsacks and @mvertens for solving the issue with bad mapunit determination in mksoiltexMod. It got me thinking—would it be helpful to add a check for bad mapping in there? It seems like mapunit can legitimately be zero over ocean, so maybe two checks:

• Initialize to a negative number, then check after mapping that it's not negative
• If it's zero after mapping, cross-reference with the land mask

Is that something we should consider?