check return status of extractSHRT() in testMatrix()

[cary-ilm]

testMatrix() in testExtractSHRT.cpp fails to check the return status of extractSHRT(). If the input matrix is invalid, it will produce garbage results. A better solution is to assert that extractSHRT() returns true.

Adding this assert will eliminate an uninitialized variable reference warning, since the s, h, r, and t variables may be unset.