Misc. fixes to compiler warnings, etc.

Marlin/src/gcode/bedlevel/abl/G29.cpp

@@ -630,6 +630,8 @@ void GcodeSuite::G29() {
 
       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++) {
 

Marlin/src/gcode/motion/G2_G3.cpp

@@ -51,6 +51,7 @@ void plan_arc(
   #if ENABLED(CNC_WORKSPACE_PLANES)
     AxisEnum p_axis, q_axis, l_axis;
     switch (gcode.workspace_plane) {
+      default:
       case GcodeSuite::PLANE_XY: p_axis = X_AXIS; q_axis = Y_AXIS; l_axis = Z_AXIS; break;
       case GcodeSuite::PLANE_ZX: p_axis = Z_AXIS; q_axis = X_AXIS; l_axis = Y_AXIS; break;
       case GcodeSuite::PLANE_YZ: p_axis = Y_AXIS; q_axis = Z_AXIS; l_axis = X_AXIS; break;