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/drawscreen.c b/src/drawscreen.c
index d55c2ea..5a23d54 100644
--- a/src/drawscreen.c
+++ b/src/drawscreen.c
@@ -2486,7 +2486,7 @@
 		// Let the syntax stuff know we skipped a few lines.
 		if (syntax_last_parsed != 0 && syntax_last_parsed + 1 < lnum
 						       && syntax_present(wp))
-		    syntax_end_parsing(syntax_last_parsed + 1);
+		    syntax_end_parsing(wp, syntax_last_parsed + 1);
 #endif
 
 		// Display one line.
@@ -2601,7 +2601,7 @@
 #ifdef FEAT_SYN_HL
     // Let the syntax stuff know we stop parsing here.
     if (syntax_last_parsed != 0 && syntax_present(wp))
-	syntax_end_parsing(syntax_last_parsed + 1);
+	syntax_end_parsing(wp, syntax_last_parsed + 1);
 #endif
 
     // If we didn't hit the end of the file, and we didn't finish the last