perf machine: Return NULL instead of null-terminating /proc/version array

perf machine: Return NULL instead of null-terminating /proc/version array

Return NULL instead of null-terminating version char array when fgets
fails due to end-of-file or error.

Signed-off-by: Donald Yandt <donald.yandt@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Fixes: 30ba5b0e66c8 ("perf machine: Null-terminate version char array upon fgets(/proc/version) error")
Link: http://lkml.kernel.org/r/20190528134128.30841-1-donald.yandt@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>