tools: hv: Check retrun value of strchr call

Check return value of strchr call and dereference it only if it is
not NULL.

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Acked-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>