tools lib traceevent: Fix reporting of unknown SVM exit reasons

tools lib traceevent: Fix reporting of unknown SVM exit reasons

On AMD, exist code -1 is also a possible value, but we use it for
terminating the list of known exit reasons. This leads to EXIT_ERR being
reported for unkown ones. Fix this by using an NULL string pointer as
terminal.

Link: http://lkml.kernel.org/r/5741D817.3070902@web.de
Link: http://lore.kernel.org/linux-trace-devel/20200702174950.123454-7-tz.stoyanov@gmail.com
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
[ Ported from trace-cmd.git ]
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20200702185705.759824282@goodmis.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>