strncat count should be free space in buffer minus terminating null byte

Index: src/ceps.c
--- src/ceps.c.orig
+++ src/ceps.c
@@ -1522,8 +1522,8 @@ write_platefile_comments(Doc *doc, GFile *docfile, 
 	    memset(platename, 0, sizeof(platename));
 	    cs_to_narrow(platename, sizeof(platename)-1, epsname, 
 		(int)cslen(epsname)+1);
-	    strncat(platename, ".", sizeof(platename) - strlen(platename));
-	    strncat(platename, sepname, sizeof(platename) - strlen(platename));
+	    strncat(platename, ".", sizeof(platename) - 1 - strlen(platename));
+	    strncat(platename, sepname, sizeof(platename) - 1 - strlen(platename));
 	    for (j=1; j<i; j++) {
 		/* Check if separation name is a duplicate */ 
 		if (strcmp(sepname, renamed[j]) == 0)
