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)