When reading file in chunks, buffer_size < dtype.itemsize, ensure at least one read
test for failing chunk read of large string
BUG: Fix chunked reading of strings longer than BUFFERSIZE

This fix correctly calculates the number of BUFFER_SIZE chunks to read.
This fix takes into account dtype sizes that could be larger
than the BUFFER_SIZE, like a long string.
See #4027
Merge pull request #4027 from halfaleague/chunk-read-bug

Chunked reading fails when dtype.itemsize is larger than buffer size.