fix angle conversion problem

pkgs/htdp-pkgs/htdp-lib/2htdp/private/img-err.rkt

@@ -349,8 +349,10 @@
                 (equal? arg +nan.0)))))
                                
 (define (angle->proper-range α)
-  (define whole-part (modulo (round α) 360))
-  (define decimal-part (- α (round α)))
+  (define eα (inexact->exact α))
+  (define rα (round eα))
+  (define whole-part (modulo rα 360))
+  (define decimal-part (- eα rα))
   (if (and (zero? whole-part)
            (negative? decimal-part))
       (+ 360 decimal-part)

pkgs/htdp-pkgs/htdp-test/2htdp/tests/test-image.rkt

@@ -996,6 +996,11 @@
       =>
       (rectangle 100 10 'solid (color 0 255 0)))
 
+;; make sure that rotation with a very small negative number doesn't crash
+(test (image? (rotate -0.0000000000000001 (rectangle 10 10 'solid 'blue)))
+      =>
+      #t)
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
 ;;  regular polygon