ftrace: Fix regression with module command in stack_trace_filter
In the Linux kernel, the following vulnerability has been resolved:

ftrace: Fix regression with module command in stack_trace_filter

When executing the following command:

    # echo "write*:mod:ext3" > /sys/kernel/tracing/stack_trace_filter

The current mod command causes a null pointer dereference. While commit
0f17976568b3f ("ftrace: Fix regression with module command in stack_trace_filter")
has addressed part of the issue, it left a corner case unhandled, which still
results in a kernel crash.