Prevent OOB in SRP base64 code.
Prevent OOB in SRP base64 code.

Change size comparison from > (GT) to >= (GTE) to ensure an additional
byte of output buffer, to prevent OOB reads/writes later in the function
Reject input strings larger than 2GB
Detect invalid output buffer size and return early

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
Reviewed-by: Rich Salz <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2672)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl