fix warning

Since SARAH 4.2 the list SARAH`Masses[EWSB] contains elements of the
form `0 -> MassGiven[VG]' for vector bosons, for examples

  {0 -> MassGiven[VG],
   Mass[gG] -> MassGiven[gG],
   Mass[Glu] -> MassRead[Glu],
   Mass[Fv[1]] -> MassGiven[Fv[1]],
   ... }

For these elements GetHead[#[[1,1]]] produces an error, since `0' is
not a list.  This commit fixes the bug by filtering only elements out
which are of the form

   SARAH`Mass[p_] -> _