[release-branch.go1.3] crypto/rsa: fix out-of-bound access with short session keys.

[release-branch.go1.3] crypto/rsa: fix out-of-bound access with short session keys.

««« CL 102670044 / c5f72a685e25
crypto/rsa: fix out-of-bound access with short session keys.

Thanks to Cedric Staub for noting that a short session key would lead
to an out-of-bounds access when conditionally copying the too short
buffer over the random session key.

LGTM=davidben, bradfitz
R=davidben, bradfitz
CC=golang-codereviews
https://golang.org/cl/102670044
»»»

TBR=rsc
CC=golang-codereviews
https://golang.org/cl/128930044