io: ReadAll: do not check for realloc in first round

io: ReadAll: do not check for realloc in first round

Refactor io.ReadAll to check for realloc of the buffer only after the
first read.

Fixes: #59702
Change-Id: I93b99139e6756f21738d47e7d9ad08e1d167258e
Reviewed-on: https://go-review.googlesource.com/c/go/+/486236
Auto-Submit: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>