DCM: prevent a segmentation fault when compass is not enabled

when compass is disabled _compass is NULL

libraries/AP_DCM/AP_DCM.cpp

@@ -195,7 +195,9 @@ AP_DCM::accel_adjust(void)
 void
 AP_DCM::matrix_reset(void)
 {
-	_compass->null_offsets_disable();		
+	if (_compass != NULL) {
+		_compass->null_offsets_disable();		
+	}
 	_dcm_matrix.a.x = 1.0f;
 	_dcm_matrix.a.y = 0.0f;
 	_dcm_matrix.a.z = 0.0f;
@@ -208,8 +210,11 @@ AP_DCM::matrix_reset(void)
 	_omega_I.x = 0.0f;
 	_omega_I.y = 0.0f;
 	_omega_I.z = 0.0f;
-	_compass->null_offsets_enable();	// This call is needed to restart the nulling
-										// Otherwise the reset in the DCM matrix can mess up the nulling
+	if (_compass != NULL) {
+		_compass->null_offsets_enable();	// This call is needed to restart the nulling
+		// Otherwise the reset in the DCM matrix can mess up
+		// the nulling
+	}
 }
 
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