The function hist_field_name() cannot handle being passed a NULL field
parameter. It should never be NULL, but due to a previous bug, NULL was
passed to the function and the kernel crashed due to a NULL dereference.
Mark Rutland reported this to me on IRC.

The bug was fixed, but to prevent future bugs from crashing the kernel,
check the field and add a WARN_ON() if it is NULL.

Link: https://lkml.kernel.org/r/20230302020810.762384440@goodmis.org

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Reported-by: NMark Rutland <mark.rutland@arm.com>
Fixes: c6afad49 ("tracing: Add hist trigger 'sym' and 'sym-offset' modifiers")
Tested-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NSteven Rostedt (Google) <rostedt@goodmis.org>