openssl srp: make index.txt parsing error more verbose

openssl srp: make index.txt parsing error more verbose

If index.txt exists but has some problems (like for example
consisting of a single \n character or number of fields wrong in one of the lines)
then openssl will just exit. This fixes it by printing an error when
load_index returns null.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15360)