patch 9.0.0074: Coverity warns for double free

Problem:    Coverity warns for double free.
Solution:   Reset cts_text_prop_count when freeing cts_text_props.
diff --git a/src/charset.c b/src/charset.c
index 8c6ac1c..f41248b 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -964,7 +964,10 @@
 {
 #ifdef FEAT_PROP_POPUP
     if (cts->cts_text_prop_count > 0)
+    {
 	vim_free(cts->cts_text_props);
+	cts->cts_text_prop_count = 0;  // avoid double free
+    }
 #endif
 }