runtime: don't panic for bad size hint in hashmap

runtime: don't panic for bad size hint in hashmap

Because the hint parameter is supposed to be treated
purely as a hint, if it doesn't meet the requirements
we disregard it and continue as if there was no hint
at all.

Fixes #19926

Change-Id: I86e7f99472fad6b99ba4e2fd33e4a9e55d55115e
Reviewed-on: https://go-review.googlesource.com/40854
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>