The function BN_MONT_CTX_set was assuming that the modulus was non-zero
and therefore that |mod->top| > 0. In an error situation that may not be
the case and could cause a seg fault.
This is a follow on from CVE-2015-1794.
(Imported from upstream's 512368c9ed4d53fb230000e83071eb81bf628b22.)
The CVE itself doesn't affect us as the bit strength check in the DHE logic
excludes zero.
Also add tests to bn_test for a couple of division by zero cases. (This and
BN_div.)
Change-Id: Ibd8ef98d6be48eb95110021c23cd8e278656764d
Reviewed-on: https://boringssl-review.googlesource.com/5690
Reviewed-by: Adam Langley <agl@google.com>
9e45d6e42f