Don't set `len` in `BytesMut::reserve` #590

Job	Run time
loom	1m 6s
miri	10m 27s
minrust	1m 3s
cross (i686-unknown-linux-gnu)	40s
cross (armv7-unknown-linux-gnueabihf)	41s
nightly	1m 59s
rustfmt	10s
cross (powerpc-unknown-linux-gnu)	42s
stable (ubuntu-latest)	1m 12s
cross (powerpc64-unknown-linux-gnu)	38s
stable (macos-latest)	1m 45s
tsan	1m 27s
cross (wasm32-unknown-unknown)	13s
stable (windows-latest)	2m 6s
Publish Documentation	50s
	24m 59s

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