Fixes for test_create_frame_rand1.py

1. Add more bounds checking for _mins.length 0 and _maxs.length 0.
Was getting an AIOOBE for synthetic frame from CreateFrame.

2. Disable some assertion checks when there is only one bucket.
GET  /2/CreateFrame.json randomize=1 integer_fraction=0 real_range=1234567890 response_factors=0 factors=1
was able to create a frame that tripped this assertion.

This may or may not be OK.

The vec thought it was isEnum() true, but the Chunk had negative double values (C8DChunk).
Merge branch 'master' of github.com:0xdata/h2o