Update submodule matrix to latest Fri Aug 31 21:31:02 UTC 2018

- matrix in PX4/Firmware (468f28463eabc30843e606546d642adc1a62fe10): b815fc97c4
    - matrix current upstream: dc3af80977
    - Changes: b815fc97c4...dc3af80977

dc3af80 2018-08-27 Daniel Agar - constructors use array size rather than pointers
f1bee77 2018-08-27 Daniel Agar - use default constructors and skip unnecessary initialization
1bcf48b 2018-08-30 Daniel Agar - Quaternion from_dcm don't pass by value

src/lib/matrix

@@ -1 +1 @@
-Subproject commit b815fc97c4e686a93a8074f27d1830a031b0d38d
+Subproject commit dc3af8097798881b9b2e72a029a3c6c10abd059b

src/systemcmds/tests/test_matrix.cpp

@@ -449,8 +449,7 @@ bool MatrixTest::matrixAssignmentTests()
 	ut_test(isEqual(m4, Matrix3f(data_row_02_swap)));
 	ut_test(fabs(m4.min() - 1) < 1e-5);
 
-	Scalar<float> s;
-	s = 1;
+	Scalar<float> s = 1;
 	ut_test(fabs(s - 1) < 1e-5);
 
 	Matrix<float, 1, 1> m5 = s;