perf machine: Null-terminate version char array upon fgets(/proc/version) error

perf machine: Null-terminate version char array upon fgets(/proc/version) error

If fgets() fails due to any other error besides end-of-file, the version
char array may not even be null-terminated.

Signed-off-by: Donald Yandt <donald.yandt@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Avi Kivity <avi@scylladb.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Fixes: cb30dcad8c90 ("perf: 'perf kvm' tool for monitoring guest performance from host")
Link: http://lkml.kernel.org/r/20190514110100.22019-1-donald.yandt@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>