h264_ps: when parsing a VUI fails, only abort when explode is set

A VUI doesn't contain anything strictly necessary for decoding.
Apparently there are many samples with truncated VUIs in the wild, this
commit should allow decoding them.
AVOptions: do not range check flag options.

It does not make sense in the vast majority of use cases, no currently
defined AV_OPT_TYPE_FLAGS options in Libav set the range to anything
nontrivial, and many of those get it wrong (the "correct" range is
INT_MIN to INT_MAX so that the builtin constant "all" works).