Eliminate some compiler warnings

Marlin/Marlin_main.cpp

 
         bool zig = PR_OUTER_END & 1;  // Always end at RIGHT and BACK_PROBE_BED_POSITION
 
+        measured_z = 0;
+
         // Outer loop is Y with PROBE_Y_FIRST disabled
         for (uint8_t PR_OUTER_VAR = 0; PR_OUTER_VAR < PR_OUTER_END && !isnan(measured_z); PR_OUTER_VAR++) {
 
     #if ENABLED(CNC_WORKSPACE_PLANES)
       AxisEnum p_axis, q_axis, l_axis;
       switch (workspace_plane) {
+        default:
         case PLANE_XY: p_axis = X_AXIS; q_axis = Y_AXIS; l_axis = Z_AXIS; break;
         case PLANE_ZX: p_axis = Z_AXIS; q_axis = X_AXIS; l_axis = Y_AXIS; break;
         case PLANE_YZ: p_axis = Y_AXIS; q_axis = Z_AXIS; l_axis = X_AXIS; break;