TMstate.c: Handle -Wduplicated-branches warnings

Gets rid of these messages from gcc:

TMstate.c: In function ‘GetBranchHead’:
TMstate.c:128:12: warning: this condition has identical branches
  [-Wduplicated-branches]
         if (parseTree->branchHeadTblSize == 0)
            ^
TMstate.c: In function ‘_XtGetQuarkIndex’:
TMstate.c:183:16: warning: this condition has identical branches
  [-Wduplicated-branches]
             if (parseTree->quarkTblSize == 0)
                ^

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>