Avoid discarding capacity during read operation

[vpzomtrrfrt]

The previous behaviour caused the target buffer length to change between calls within a single read operation, which seems wrong since the underlying read would then try to read too much into a smaller buffer

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 65.21739% with 8 lines in your changes missing coverage. Please review.

Project coverage is 88.5%. Comparing base (85be6f1) to head (39e826d).

Files with missing lines	Patch %	Lines
src/reader.rs	65.2%	8 Missing :warning:

Additional details and impacted files

Files with missing lines	Coverage Δ
src/reader.rs	84.4% <65.2%> (-5.2%)	:arrow_down:

[jonhoo]

Published as 0.7.3 :tada: