Merge pull request #794 from MissionSt/g29_requires_home

Prevent G29 without first homing in X and Y

Marlin/Marlin_main.cpp

 // G10 - retract filament according to settings of M207
 // G11 - retract recover filament according to settings of M208
 // G28 - Home all Axis
-// G29 - Detailed Z-Probe, probes the bed at 3 points.  You must de at the home position for this to work correctly.
+// G29 - Detailed Z-Probe, probes the bed at 3 or more points.  Will fail if you haven't homed yet.
 // G30 - Single Z Probe, probes bed at current XY location.
 // G90 - Use Absolute Coordinates
 // G91 - Use Relative Coordinates
             #error "You must have a Z_MIN endstop in order to enable Auto Bed Leveling feature!!! Z_MIN_PIN must point to a valid hardware pin."
             #endif
 
+            // Prevent user from running a G29 without first homing in X and Y
+            if (! (axis_known_position[X_AXIS] && axis_known_position[Y_AXIS]) )
+            {
+                LCD_MESSAGEPGM(MSG_POSITION_UNKNOWN);
+                SERIAL_ECHO_START;
+                SERIAL_ECHOLNPGM(MSG_POSITION_UNKNOWN);
+                break; // abort G29, since we don't know where we are
+            }
+
             st_synchronize();
             // make sure the bed_level_rotation_matrix is identity or the planner will get it incorectly
             //vector_3 corrected_position = plan_get_position_mm();