patch 8.0.1620: reading spell file has no good EOF detection

Problem:    Reading spell file has no good EOF detection.
Solution:   Check for EOF at every character read for a length field.
diff --git a/src/version.c b/src/version.c
index 6ac9000..acc535e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1620,
+/**/
     1619,
 /**/
     1618,