isConstructorType checks base constraint for undefined

Previously, it assumed there was always a base constraint, which is true
for correct code. For incorrect code, the base constraint may be
missing, which caused a crash because the base constraint was undefined.

src/compiler/checker.ts

@@ -3952,7 +3952,7 @@ namespace ts {
             }
             if (type.flags & TypeFlags.TypeVariable) {
                 const constraint = getBaseConstraintOfType(<TypeVariable>type);
-                return isValidBaseType(constraint) && isMixinConstructorType(constraint);
+                return constraint && isValidBaseType(constraint) && isMixinConstructorType(constraint);
             }
             return false;
         }