perf report: Fix segfault on unknown symbols

Ingo reported:

> Program received signal SIGSEGV, Segmentation fault.
> 0x0000003e25080f80 in strlen () from /lib64/libc.so.6
> Missing separate debuginfos, use: debuginfo-install elfutils.x86_64
> glibc.x86_64 zlib.x86_64
> (gdb) bt
> #0  0x0000003e25080f80 in strlen () from /lib64/libc.so.6
> #1  0x0000003e2506954e in fputs () from /lib64/libc.so.6
> #2  0x00000000004059e8 in cmd_report (argc=<value optimized out>,
>     argv=<value optimized out>) at builtin-report.c:521
> #3  0x0000000000402dad in handle_internal_command (argc=1, argv=0x7fffe1218e30)
>     at perf.c:226
> #4  0x0000000000402f6d in main (argc=1, argv=0x7fffe1218e30) at perf.c:324
> (gdb)

Signed-off-by Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/perf_counter/builtin-report.c

@@ -518,7 +518,7 @@ static size_t symhist__fprintf(struct symhist *self, FILE *fp)
 	size_t ret = fprintf(fp, "%#llx [%c] ", (unsigned long long)self->ip, self->level);
 
 	if (self->level != '.')
-		ret += fprintf(fp, "%s", self->sym->name);
+		ret += fprintf(fp, "%s", self->sym ? self->sym->name: "<unknown>");
 	else
 		ret += fprintf(fp, "%s: %s",
 			       self->dso ? self->dso->name : "<unknown",