Check the consistency of number of items in metadata and data in dry_load_files function

[dachengx]

What is the problem / what does the code in this PR do

In strax/storage/common.py, the number of items in metadata and data is checked:

strax/strax/storage/common.py

Lines 532 to 536 in 208d530

	if len(data) != chunk_info["n"]:
	raise strax.DataCorrupted(
	f"Chunk {chunk_info['filename']} of {chunk_info['run_id']} has {len(data)} items, "
	f"but chunk_info {chunk_info} says {chunk_info['n']}"
	)

This PR also checks this in dry_load_files function

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: 91.504% (+0.01%) from 91.494%
when pulling 179ede4 on check_n_dry_load_files
into 208d530 on master.