C++: Reduce number of FPs cpp/guarded-free and turn if(x) { free(x) } cases from FNs to TPs

[jketema]

The additional conditions to reduce FPs are maybe somewhat on the strict side, but given that a MRVA 1000 experiment already gives over 5k results, I don't think that this much of a problem. As far as I can tell there is just one remaining FP in MRVA (which occurs a few times), and which comes from the libpng library.

https://github.com/naturalatlas/node-gdal/blob/c83e7858a9ec566cc91d65db74fd07b99789c0f0/deps/libgdal/gdal/frmts/png/libpng/pngmem.c#L541-L560

Given that this is now the only FP, I did not investigate this further. I think we could promote the query with these changes from this PR.

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