rtla/utils: Fix loop condition in PID validation

The procfs_is_workload_pid() function iterates through a directory
entry name to validate if it represents a process ID. The loop
condition checks if the pointer t_name is non-NULL, but since
incrementing a pointer never makes it NULL, this condition is always
true within the loop's context. Although the inner isdigit() check
catches the NUL terminator and breaks out of the loop, the condition
is semantically misleading and not idiomatic for C string processing.

Correct the loop condition from checking the pointer (t_name) to
checking the character it points to (*t_name). This ensures the loop
terminates when the NUL terminator is reached, aligning with standard
C string iteration practices. While the original code functioned
correctly due to the existing character validation, this change
improves code clarity and maintainability.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Link: https://lore.kernel.org/r/20260309195040.1019085-19-wander@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>