Don't attempt to write on invalid input

[pramsey]

Not sure what exact input causes this, but it's necessary to regress PostGIS on all platforms

[rouault]

The commit message doesn't match what the code does. What are the situations where p == NULL or nmemb == 0 ?

[pramsey]

Just blindly getting our software running again... https://github.com/postgis/postgis/commit/00eb4b44b4e8b7c98d8b9cbc611aa496812b33ee#diff-7dca6e3da1ca13b272669d5646bcf00bL118

[r-barnes]

This looks reasonable to me.

[thbeu]

@rouault Once again, I propose to close this PR without merging.