uninitialized string in xinfix

Glenn Morris reported bizarre behaviour in xspec11 when giving setplot an invalid argument. This turned out to be an uninitialized string in xanlib/xparse/xinfix.f which has somehow avoided detection before now. Note that we really need to fix the Makefile in xspec11/src/tcl so that libxantcl*.a is rebuilt if libxanlib*.a is changed.