Explicitly set pointer to NULL

[kstribrnAmzn]

Description

Fixing test cases by explicitly setting
pContext pointer to NULL for later check.

Testing

Checked out OTA package, built unit tests, observed two failing tests.
Build commands:

cmake -S test -B build
make -C build all
cd build && ctest

Previously failing tests:

Test project /home/kstribrn/workspace/ota-for-aws-iot-embedded-sdk/build
    Start 1: ota_utest
1/4 Test #1: ota_utest ........................***Failed    0.02 sec
    Start 2: ota_base64_utest
2/4 Test #2: ota_base64_utest .................   Passed    0.00 sec
    Start 3: ota_cbor_utest
3/4 Test #3: ota_cbor_utest ...................   Passed    0.00 sec
    Start 4: ota_os_posix_utest
4/4 Test #4: ota_os_posix_utest ...............   Passed    5.01 sec

75% tests passed, 1 tests failed out of 4

Total Test time (real) =   5.03 sec

The following tests FAILED:
	  1 - ota_utest (Failed)
Errors while running CTest

/home/kstribrn/workspace/ota-for-aws-iot-embedded-sdk/test/unit-test/ota_utest.c:3603:test_OTA_parseJobFailsMoreBlocksThanBitmap:FAIL: Expected NULL
/home/kstribrn/workspace/ota-for-aws-iot-embedded-sdk/test/unit-test/ota_utest.c:3623:test_OTA_extractParameterFailInvalidJobDocModel:FAIL: Expected NULL
-----------------------
150 Tests 2 Failures 0 Ignored 

Test results now:

Test project /home/kstribrn/workspace/ota-for-aws-iot-embedded-sdk/build
    Start 1: ota_utest
1/4 Test #1: ota_utest ........................   Passed    0.02 sec
    Start 2: ota_base64_utest
2/4 Test #2: ota_base64_utest .................   Passed    0.00 sec
    Start 3: ota_cbor_utest
3/4 Test #3: ota_cbor_utest ...................   Passed    0.00 sec
    Start 4: ota_os_posix_utest
4/4 Test #4: ota_os_posix_utest ...............   Passed    5.00 sec

100% tests passed, 0 tests failed out of 4

Total Test time (real) =   5.03 sec

Checklist:

• I have tested my changes. No regression in existing tests.
• My code is formatted using Uncrustify.
• I have read and applied the rules stated in CONTRIBUTING.md.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.