Cause compiler error when (if ...) has spurious args

scheme-core/compiler.scm

@@ -262,9 +262,12 @@
         (endl  (make-label g))
         (test  (cadr x))
         (then  (caddr x))
-        (else  (if (pair? (cdddr x))
-                   (cadddr x)
-                   (void))))
+        (else  (cond ((null? (cdddr x))
+                      (void))
+                     ((null? (cddddr x))
+                      (cadddr x))
+                     (else
+                      (error "compile error: if expects 2-3 arguments.")))))
     (cond ((eq? test #t)
            (compile-in g env tail? then))
           ((eq? test #f)