AP_Compass: fixed zero compass diagonals

this fixes a regression from 4.2 to 4.3.

previously we automatically set the diagnoals to 1,1,1 if they were
0,0,0. We don't do that any more. I was helping a user who had copied
an old config with 0,0,0 for diagonals and did not understand two
things:

- the compass did not work in 4.3
- large vehicle mag cal didn't work

libraries/AP_Compass/AP_Compass_Backend.cpp

@@ -87,6 +87,7 @@ void AP_Compass_Backend::correct_field(Vector3f &mag, uint8_t i)
     }
 
     // apply eliptical correction
+    if (!diagonals.is_zero()) {
         Matrix3f mat(
             diagonals.x,    offdiagonals.x, offdiagonals.y,
             offdiagonals.x, diagonals.y,    offdiagonals.z,
@@ -94,6 +95,7 @@ void AP_Compass_Backend::correct_field(Vector3f &mag, uint8_t i)
             );
 
         mag = mat * mag;
+    }
 
 #if COMPASS_MOT_ENABLED
     const Vector3f &mot = state.motor_compensation.get();

libraries/AP_Compass/AP_Compass_Calibration.cpp

@@ -450,7 +450,12 @@ bool Compass::get_uncorrected_field(uint8_t instance, Vector3f &field) const
     // form eliptical correction matrix and invert it. This is
     // needed to remove the effects of the eliptical correction
     // when calculating new offsets
+
+    // get corrected field
+    field = get_field(instance);
+
     const Vector3f &diagonals = get_diagonals(instance);
+    if (!diagonals.is_zero()) {
         const Vector3f &offdiagonals = get_offdiagonals(instance);
         Matrix3f mat {
             diagonals.x, offdiagonals.x, offdiagonals.y,
@@ -461,11 +466,9 @@ bool Compass::get_uncorrected_field(uint8_t instance, Vector3f &field) const
             return false;
         }
 
-    // get corrected field
-    field = get_field(instance);
-
         // remove impact of diagonals and off-diagonals
         field = mat * field;
+    }
 
     // remove impact of offsets
     field -= get_offsets(instance);