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/memline.c b/src/memline.c
index 46e7d8a..3483f09 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -3626,7 +3626,7 @@
     int		idx;
     int		line_start;
     long	line_size;
-    int		this_props_len;
+    int		this_props_len = 0;
     char_u	*text;
     size_t	textlen;
     int		found;