rtla/timerlat: Add bounds check for softirq vector

Add bounds checking when accessing the softirq_name array using the
vector value from kernel trace data. The vector field from the
osnoise:softirq_noise event is used directly as an array index without
validation, which could cause an out-of-bounds read if the kernel
provides an unexpected vector value.

The softirq_name array contains 10 elements corresponding to the
standard Linux softirq vectors. While the kernel should only provide
valid vector values in the range 0-9, defensive programming requires
validating untrusted input before using it as an array index. If an
out-of-range vector is encountered, display the word UNKNOWN instead
of attempting to read beyond the array bounds.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Link: https://lore.kernel.org/r/20260309195040.1019085-9-wander@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>