Improve fuzzing

[NikolajSchlej]

I procrastinated from writing the builder (that I promised to restart writing in 2022 for the third time) by fixing all P1s, and the last remaining one was the lack of fuzzing target, that I've added in 209fbb6.
That trivial piece of code and some assistance from HonggFuzz uncovered a giant pile of crashes that I will use this issue to discuss and fix.

[NikolajSchlej]

Might also use this issue to try other fuzzers:

• AFL++
• AFLSmart

[NikolajSchlej]

Thanks to the good folks from Binarly (@pagabuc and @yeggor), this issue can be reduced from L0 to L2, as the all the immediate crashes found by fuzzing had been fixed.

[NikolajSchlej]

#463