avoid problems testing dihedrals and impropers

unittest/force-styles/tests/in.fourmol

@@ -28,6 +28,5 @@ dihedral_style   ${dihedral_style}
 improper_style   ${improper_style}
 
 read_data        ${data_file}
-dihedral_coeff   *
+if "${dihedral_style} == zero" then "dihedral_coeff *"
-improper_coeff   *
+if "${improper_style} == zero" then "improper_coeff *"
-