Removed an assertion in msgpack

[AdamKorcz]

I ran the tests, and the results were the same before and after this update

[vstakhov]

I'm not quite sure that this particular assertion should be removed. Assertions are used to prevent library misusage. A missing parser object is definitely an example of misusage.

Assertions on bad inputs should definitely be removed, on the contrary.

[AdamKorcz]

I agree.
We can't, however, fuzz the msgpack functionality, if misusage causes our fuzzers to stop completely in an abort. The rewrite from an abort to the ucl_create_err() will allow our fuzzers to run continuously.
I should note that we most likely need this to be done on all aborts in the msgpack-functionality, so if a few ones are essential then it won't be necessary to rewrite the rest.