Add check for mixed genotype so that we don't exception out for a valid record

public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/evaluators/CountVariants.java

@@ -182,6 +182,8 @@ public class CountVariants extends VariantEvaluator implements StandardEval {
                         nHomDerived++;
                     }
 
+                    break;
+                case MIXED:
                     break;
                 default:
                     throw new ReviewedStingException("BUG: Unexpected genotype type: " + g);