crypto/rsa: reject invalid length PKCS#1v1.5 signatures

Per RFC 8017, reject signatures which are not the same length as the RSA
modulus. This matches the behavior of SignPKCS1v15 which properly left pads
the signatures it generates to the size of the modulus.

Fixes #21896

Change-Id: I2c42a0b24cf7fff158ece604b6f0c521a856d932
GitHub-Last-Rev: 6040f7990633630a0ad157cb17e016bb7db98a27
GitHub-Pull-Request: golang/go#38140
Reviewed-on: https://go-review.googlesource.com/c/go/+/226203


Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>