BN_mod_exp(r,a,p,m,ctx) should not be called with r == p.

BN_mod_exp(r,a,p,m,ctx) should not be called with r == p.
But even if this is avoided, there are still segmentation violations
(during one of the BN_free()s at the end of test_kron
in some cases, in other cases during BN_kronecker, or
later in BN_sqrt; choosing a different exponentiation
algorithm in bntest.c appears to influence when the SIGSEGV
takes place).