Fix unchecked calls to ne_ctx_push().
Fix unchecked calls to ne_ctx_push().

nestegg_init() and ne_match_webm() called ne_ctx_push() without checking
its return value.  If "item = ne_alloc(sizeof(*item))" failed, the
subsequent call to ne_parse() would die at "assert(ctx->ancestor)".

kinetiknz/nestegg,kinetiknz/nestegg,kinetiknz/nestegg