drm/xe/xe_devcoredump: Check NULL before assignments

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)

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240328123739.3633428-1-himal.prasad.ghimiray@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>