strconv: cleanup variable declarations in ParseUint

strconv: cleanup variable declarations in ParseUint

Move variable declarations closer to their first uses.

Use an additional string variable s0 for error reporting that references
the original input string. This allows the variable s to be modified.

Change-Id: I4725152490ca1dc10c1161ad8ad2f4ae8933493f
Reviewed-on: https://go-review.googlesource.com/55138
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>