patch 9.0.0501: warning for using uninitialized value in mouse test

Problem:    Warning for using uninitialized value in mouse test.
Solution:   Clear ScreenCols when allocating it. (Dominique Pellé)
diff --git a/src/version.c b/src/version.c
index a167c38..46e7099 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    501,
+/**/
     500,
 /**/
     499,