Fix: copy_a deallocation in determinant

[perazz]

I noticed a typo in det that does not deallocate the internal pointer matrix properly: it should be when the temporary is created (copy_a==.true.):

stdlib/src/stdlib_linalg_determinant.fypp

Line 227 in 206f84a

if (.not.copy_a) deallocate(amat)

I don't know why an error was never printed in the tests, as it should be impossible to deallocate the target
of a pointer, if this is not allocatable.
Anyways this is a fix for it, plus an additional test.

cc: @jvdp1 @jalvesz

[jvdp1]

LGTM. Thank you @perazz

[perazz]

Thank you @jvdp1. I will merge this as it was a clear bug.