drm/xe/xe_devcoredump: Check NULL before assignments

In the Linux kernel, the following vulnerability has been resolved:

drm/xe/xe_devcoredump: Check NULL before assignments

Assign 'xe_devcoredump_snapshot *' and 'xe_device *' only if

'coredump' is not NULL.

v2

- Fix commit messages.

v3

- Define variables before code.(Ashutosh/Jose)

v4

- Drop return check for coredump_to_xe. (Jose/Rodrigo)

v5

- Modify misleading commit message. (Matt)