patch 8.0.0333: illegal memory access when 'complete' ends in a backslash

Problem:    Illegal memory access when 'complete' ends in a backslash.
Solution:   Check for trailing backslash. (Dominique Pelle, closes #1478)
diff --git a/src/option.c b/src/option.c
index 32e97c9..b5f3a9a 100644
--- a/src/option.c
+++ b/src/option.c
@@ -7017,7 +7017,7 @@
 		    /* skip optional filename after 'k' and 's' */
 		    while (*s && *s != ',' && *s != ' ')
 		    {
-			if (*s == '\\')
+			if (*s == '\\' && s[1] != NUL)
 			    ++s;
 			++s;
 		    }