Fix of handling weak dh server keys.

https://crbug.com/353579

Align behavior with NSS and report SSL_R_BAD_DH_P_LENGTH error
when size of the server's dh group is less than 512 bits.

Change-Id: I09f1828482f40b2283f7c6a69425819379399815
Reviewed-on: https://boringssl-review.googlesource.com/1480
Reviewed-by: Adam Langley <agl@google.com>