With some (buggy) drivers, the VIDIOC_G_STD ioctl returns a std_id that cannot
be matched with any of the enumerated v4l2_standard structures (for example
std_id = 0 or std_id = 0xffffff). Do not fail when we reach the end of the
enumeration without a valid match.

Fixes ticket #2370

Note: This commit message has been modified by Giorgio Vazzana, the original
commit message was:

"Fixed regression for mandatory VIDIOC_ENUMSTD support by v4l2"
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>