Prevent start being negative

[dachengx]

What is the problem / what does the code in this PR do
Negative start is not compatible with

strax/strax/plugins/overlap_window_plugin.py

Line 25 in 9454279

self.sent_until = 0

and

strax/strax/chunk.py

Line 270 in 9454279

prev_end = 0

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

coverage: 91.196% (+0.002%) from 91.194%
when pulling ba97ac2 on no_negative_start
into 9454279 on master.

[coveralls]

coverage: 91.196% (+0.002%) from 91.194%
when pulling ba97ac2 on no_negative_start
into 9454279 on master.

[coveralls]

coverage: 91.196% (+0.002%) from 91.194%
when pulling ba97ac2 on no_negative_start
into 9454279 on master.