the switcap backend of gnetlist crashes in case of non-existent file

[Vladimir Zhbanov]

The switcap backend crashes if the required analysis file
specified in the attribute "file=" of the symbol
"switcap-analysis-1.sym" is missing or its name is spelled
wrongly.

How to reproduce:
1. cd to the ./gnetlist/examples/switcap/ directory in the
geda-gaf repository
2. 'mv test.ana test1.ana' or open analysis.sch and change the
filename in the Switcap Analysis Block.
3. Run 'gnetlist -g switcap -o example.scn *.sch'

Attached is a patch to fix this.

[Edward Hennessy (ehennes)]

Thank you for the patch and instructions to reproduce the defect.