C++: Fix FP in cpp/incorrectly-checked-scanf

[MathiasVP]

Note that, because the cpp/missing-check-scanf query only raises an alert if cpp/incorrectly-checked-scanf doesn't, by fixing the FP in cpp/incorrectly-checked-scanf we're now moving the alert to the cpp/missing-check-scanf query.

This is a good trade-off since cpp/incorrectly-checked-scanf is a high-precision query, and cpp/missing-check-scanf is only at precision medium.

I tried to fix the FP in cpp/incorrectly-checked-scanf as well, but this is more difficult. So I think we should get this fix in first before trying to fix the cpp/incorrectly-checked-scanf case.

[jketema]

LGTM if DCA is ok.