RFR: 1040: Crash in webrev command

[Erik Joelsson]

Validate the file input to GitWebrev.

-------------

Commit messages:
 - SKARA-1040

Changes: https://git.openjdk.java.net/skara/pull/1199/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1199&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1040
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1199.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1199/head:pull/1199

PR: https://git.openjdk.java.net/skara/pull/1199