MIPS: ftrace: Remove pointer comparison to 0 in prepare_ftrace_return

Replace pointer comparison to 0 with NULL in prepare_ftrace_return
to improve code readability. Identified with coccinelle script
'badzero.cocci'.
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18494/Signed-off-by: James Hogan <jhogan@kernel.org>

arch/mips/kernel/ftrace.c

@@ -361,7 +361,7 @@ void prepare_ftrace_return(unsigned long *parent_ra_addr, unsigned long self_ra,
 	 * If fails when getting the stack address of the non-leaf function's
 	 * ra, stop function graph tracer and return
 	 */
-	if (parent_ra_addr == 0)
+	if (parent_ra_addr == NULL)
 		goto out;
 #endif
 	/* *parent_ra_addr = return_hooker; */