XkbUpdateKeyTypeVirtualMods: always initialize mask

XkbVirtualModsToReal should only fail to set mask if the server does
not support XKB, but it still made Oracle Parfait complain:

Error: Uninitialised memory
   Uninitialised memory variable [uninitialised-mem-var] (CWE 457):
      Possible access to uninitialised memory referenced by variable 'mask'
        at line 863 of lib/libX11/src/xkb/XKBMisc.c in function 'XkbUpdateKeyTypeVirtualMods'.
        Path in callee avoiding write at line 862
          mask allocated at line 860

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
6 jobs for parfait in 7 minutes and 24 seconds