_GoStringLen performs an implicit conversion from intgo to size_t.
Explicitly cast to size_t.

This change avoids warnings when using cgo with CFLAGS:
-Wconversion.

Change-Id: I58f75a35e17f669a67f9805061c041b03eddbb5c
GitHub-Last-Rev: b5df1ac0c3c90360fa1d22c069e0f126e9f894d8
GitHub-Pull-Request: golang/go#27092
Reviewed-on: https://go-review.googlesource.com/129820
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>