tty: n_smux: fix test pattern validation

Incorrect test pattern code was skipping the test pattern validation
test and not freeing the test buffer resulting a slow memory leak in the
unit test that could lead to out-of-memory failures after several
hundred thousand iterations.

Fix pointer test to properly free memory after test case completion and
fix incorrect test pattern validation code.

CRs-Fixed: 493907
Change-Id: If93c2cfd2f82295897b36ffaf98ecaa5b9128cfb
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>
Signed-off-by: Sridhar Gujje <sgujje@codeaurora.org>