net/textproto: eliminate some bounds checks

[jub0bs]

This change lifts bounds checks out of loops in the trim function.
Here are some benchmark results (no change to allocations):

goos: darwin
goarch: amd64
pkg: net/textproto
cpu: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
│ old │ new │
│ sec/op │ sec/op vs base │
ReadMIMEHeader/client_headers-8 3.531µ ± 3% 3.506µ ± 1% ~ (p=0.068 n=20)
ReadMIMEHeader/server_headers-8 2.409µ ± 0% 2.384µ ± 1% -1.02% (p=0.000 n=20)
Uncommon-8 545.2n ± 1% 536.7n ± 0% -1.58% (p=0.000 n=20)
geomean 1.667µ 1.649µ -1.10%

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Patch Set 1:

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Patch Set 1:

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Patch Set 1: Auto-Submit+1 Code-Review+2 Run-TryBot+1

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