This just tries to convince MSVC that we're not going to use |nprimes|
without initialising it first:

tool\genrsa.cc(63) : warning C4701: potentially uninitialized local
variable 'nprimes' used

Change-Id: If8a68ad4fe2c2fb7a8073b7ba43d540467ddf8f8

RSA with more than two primes is specified in
https://tools.ietf.org/html/rfc3447, although the idea goes back far
earier than that.

This change ports some of the changes in
http://rt.openssl.org/Ticket/Display.html?id=3477&user=guest&pass=guest
to BoringSSL—specifically those bits that are under an OpenSSL license.

Change-Id: I51e8e345e2148702b8ce12e00518f6ef4683d3e1
Reviewed-on: https://boringssl-review.googlesource.com/4870
Reviewed-by: Adam Langley <agl@google.com>