Security tokens: reject unsupported paths when resolving keyfiles

Token::getTokenKeyfile returned null for a path that is neither a
PKCS#11 nor an EMV token path, and every caller dereferenced the
result. Keyfile application validates the path beforehand, but the
text-mode export does not, so entering an ordinary file path at the
--export-token-keyfile prompt crashed VeraCrypt. Throw
InvalidSecurityTokenKeyfilePath instead, which the user interfaces
already report.

src/Common/Token.cpp

@@ -109,6 +109,10 @@ namespace VeraCrypt
 			}		
 		}
 
+		// Callers use the keyfile directly, so any other path is an error.
+		if (!tokenKeyfile)
+			throw InvalidSecurityTokenKeyfilePath();
+
 		return tokenKeyfile;
 	}
 }