cmParseCoberturaCoverage: Initialize CurFileName to empty string

When the file is not found in $CMAKE_SOURCE_DIR/<file path>,
the CurFileName pointer remained set to the previous file.  This
caused the new file's coverage data to populate the into the
wrong object giving incorrect results and occasionally resulting
in a seg fault.
Merge topic 'fix-cobertura-parsing' into next

1c838add cmParseCoberturaCoverage: Initialize CurFileName to empty string