Fixed an old cut-and-paste typo: change_delay should be change_timeout.

diff --git a/coders/png.c b/coders/png.c
index c701d00..76f1dd4 100644
--- a/coders/png.c
+++ b/coders/png.c
@@ -5562,7 +5562,7 @@
                         else
                           frame_timeout=PNG_UINT_31_MAX;
 
-                        if (change_delay == 2)
+                        if (change_timeout == 2)
                           default_frame_timeout=frame_timeout;
 
                         p+=4;