drm/log: Fix out-of-bounds read on empty message length

drm/log: Fix out-of-bounds read on empty message length

drm_log_draw_kmsg_record() accesses s[len - 1] to strip the trailing
newline, but len is unsigned int. If len is 0, the subtraction wraps
to UINT_MAX, causing an out-of-bounds read.

Add an early return when len is 0.

Fixes: 25e2c2a3eff5 ("drm/log: Color the timestamp, to improve readability")
Signed-off-by: Shixiong Ou <oushixiong@kylinos.cn>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260729084520.688087-1-oushixiong1025@163.com
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>