Coverity fixes for pkg_editor_test.cpp

[tylerzhao7684]

Fixed the following coverity issues:

• Adding assert to make sure the result is nullptr before returning in TEST(create, bad_flags)
• Change strncmp(buf, file1, strlen(file0) to strncmp(buf, file1, strlen(file1) to avoid copy paste error in TEST(sample_file, update_same_size)
• Add assert(fd > 0) and assert(old_stdin > 0) to make sure positive parameter is passed to dup() and dup2() in TEST(package, unpack_buffer_stdin)

[zibaiwan]

Thanks @tylerzhao7684 . The code change looks good to me. Two nitpicks.

1. Can you squash the two commits into 1 commit? You can use rebase/reset to do it, then you need a force push. The reason is the Runtime repo doesn't squash the commits into the main branch once merge, instead we allow multiple commits as it would be beneficial in some scenarios where the PR does a few different things to make the reading easier or make the later cherry-picking/revert easier. In this specific PR, 1 commit is good.

2. Can you change the commit message body (currently body is None) to include the good information that you posted in the PR? So the commit itself will have all the info you posted in the PR, since the PR info could be lost, especially in the upstream.