Fixed cached image filename generated with invalid path extensions

@@ -171,10 +171,11 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
     }
     unsigned char r[CC_MD5_DIGEST_LENGTH];
     CC_MD5(str, (CC_LONG)strlen(str), r);
+    NSURL *keyURL = [NSURL URLWithString:key];
+    NSString *ext = keyURL ? keyURL.pathExtension : key.pathExtension;
     NSString *filename = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%@",
                           r[0], r[1], r[2], r[3], r[4], r[5], r[6], r[7], r[8], r[9], r[10],
-                          r[11], r[12], r[13], r[14], r[15], [key.pathExtension isEqualToString:@""] ? @"" : [NSString stringWithFormat:@".%@", key.pathExtension]];
-
+                          r[11], r[12], r[13], r[14], r[15], ext.length == 0 ? @"" : [NSString stringWithFormat:@".%@", ext]];
     return filename;
 }