buffer overflow fix

in MULTIPLE_EXTERNAL_IP code which is not used normally...

miniupnpd/miniupnpd.c

@@ -1503,7 +1503,7 @@ init(int argc, char * * argv, struct runtime_vars * v)
 #else	/* #ifndef MULTIPLE_EXTERNAL_IP */
 			if(i+2 < argc)
 			{
-				char *val=calloc((strlen(argv[i+1]) + strlen(argv[i+2]) + 1), sizeof(char));
+				char *val = calloc((strlen(argv[i+1]) + strlen(argv[i+2]) + 2), sizeof(char));
 				if (val == NULL)
 				{
 					fprintf(stderr, "memory allocation error for listen address storage\n");