add some asserts for earlier fail when using fixed vars in MIR of optcuts

- check earlier that variables problem index is nonnegative
- MIR routine seems to require that variables are active

Author: svigerske

src/scip/benderscut_opt.c

@@ -309,6 +309,7 @@ SCIP_RETCODE computeMIRForOptimalityCut(
    for( i = 0; i < nvars; i++ )
    {
       rowinds[i] = SCIPvarGetProbindex(vars[i]);
+      assert(rowinds[i] >= 0);
       rowvals[i] = -vals[i];
    }
 

src/scip/cuts.c

@@ -9275,6 +9275,7 @@ SCIP_RETCODE constructSNFRelaxation(
 
       probidx = rowinds[i];
       /* variable should be binary */
+      assert(probidx >= 0);
       assert(probidx < nbinvars);
 
       /* binary variable was processed together with a non-binary variable */