Only read first password from file

Read just the first password and ignore anything else in the password
file. This allows you to reuse a password file from the server that also
includes a view-only password.

This fixes a regression introduced in b99daad.

vncviewer/UserDialog.cxx

@@ -109,7 +109,7 @@ void UserDialog::getUserPasswd(bool secure_, std::string* user,
   }
 
   if (!user && passwordFileName[0]) {
-    std::vector<uint8_t> obfPwd(256);
+    std::vector<uint8_t> obfPwd(8);
     FILE* fp;
 
     fp = fopen(passwordFileName, "rb");