patch 9.0.1666: compiler may warn for uninitialized variable

Problem:    Compiler may warn for uninitialized variable.
Solution:   Initialize this_props_len. (Christian Brabandt, closes #12599)
diff --git a/src/version.c b/src/version.c
index 9c451ae..948a948 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1666,
+/**/
     1665,
 /**/
     1664,