MiniSat: Be more careful about running proof code (#1982)

MiniSat: Be more careful about running proof code (#1982)

In `addClause_()`, we were checking the condition `ca[confl].size() ==
1` regardless if proofs were enabled or not, even though both branches
of the if statement only do something when proofs are enabled. This lead
to issue #1978 occurring even when not generating proofs. Note: This
commit is *not* a fix for #1978 but it makes sure that the issue does
not occur when not generating proofs/unsat cores.