Invalid parameters passed to the SRP code can be overrun an internal
buffer. Add sanity check that g, A, B < N to SRP code.
Thanks to Sean Devlin and Watson Ladd of Cryptography Services, NCC
Group for reporting this issue.
Submitted by: Bruce Stephens <bruce.stephens@isode.com>
Use same construct for EXHEADER in srp/Makefile as other makefiles to cope
with possibly empty EXHEADER.