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.

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1620,
 /**/
     1619,
 /**/