crypto/rsa: return err when key too small to compute salt for RSA PSS

crypto/rsa: return err when key too small to compute salt for RSA PSS

When PSSSaltLengthAuto is passed to SignPSS, and the key size is too
small to create a valid salt, return ErrMessageTooLong

Change-Id: I4e0d70bdd54fcd667eae10e0a70b4f540a4ebe93
Reviewed-on: https://go-review.googlesource.com/c/go/+/450796
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Roland Shoemaker <roland@golang.org>