Vasp: Fix issue with slash in POTCAR name

[jan-janssen]

Fixing the bug discussed in https://www.vasp.at/forum/viewtopic.php?t=19113

[coveralls]

Pull Request Test Coverage Report for Build 11074113279

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 70.897%

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Totals
Change from base Build 11002650205:	0.0%
Covered Lines:	10777
Relevant Lines:	15201

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💛 - Coveralls