patch 9.0.0750: crash when popup closed in callback

Problem:    Crash when popup closed in callback. (Maxim Kim)
Solution:   In syntax_end_parsing() check that syn_block is valid.
diff --git a/src/version.c b/src/version.c
index 708e762..bd9915a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    750,
+/**/
     749,
 /**/
     748,