Fix gcc warnings in core_http_client.c. Update ci.yml.

[paulbartell]

Fix gcc warnings in http_parser and core_http_client.c.

Update ci.yml to use the latest ubuntu image and -Werror and -fsanitize.

[paulbartell]

Unless there is absolute necessity, HTTP parser should not be updated. It should be also tested before updating.

The change itself is very minimal and is just to quiet a gcc warning. It is absolutely necessary to do so or we can't enable -Werror and will continue to miss compiler warnings. Here's one of the warnings generated by the library:

/project/aws-iot-device-sdk-embedded-C/libraries/standard/coreHTTP/source/dependency/3rdparty/http_parser/http_parser.h:286:37: error: comma at end of enumerator list [-Werror=pedantic]
  286 | #define HTTP_ERRNO_GEN(n, s) HPE_##n,