cleanup all uses of fclose() in vtkSTLReader, for bug #14515

Based on patch from Adrian Winterstein.

Now, the file is only closed in the function where it was opened before (RequestData()).
Instead of closing it sometimes inside a subfunction.

Also moved some variable declarations closer to usage.

Change-Id: Ic2a222a879e9a60164bcc83d1a383e0ba8e02683
Merge topic 'bug14515' into master

e579693c cleanup all uses of fclose() in vtkSTLReader, for bug #14515