We only need to check that the constructor doesn't clear errno, so
there's no need to use an invalid FILE* for that.

libstdc++-v3/ChangeLog:

	PR libstdc++/98001
	* testsuite/ext/stdio_filebuf/char/79820.cc: Do not pass invalid
	FILE* to constructor.

Tested powerpc64le-linux. Committed to trunk.