h264: return meaningful values

Signed-off-by: Anton Khirnov <anton@khirnov.net>
h264: use explicit variable names for *_field_flag

Signed-off-by: Anton Khirnov <anton@khirnov.net>
h264: check one context_init() allocation

Signed-off-by: Anton Khirnov <anton@khirnov.net>