[script] Avoid trying to read final character of a zero-length string

Detected using Valgrind.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/image/script.c

 		script_offset += ( frag_len + 1 );
 
 		/* Strip trailing CR, if present */
-		if ( line[ len - 1 ] == '\r' )
+		if ( len && ( line[ len - 1 ] == '\r' ) )
 			len--;
 
 		/* Handle backslash continuations */
-		if ( line[ len - 1 ] == '\\' ) {
+		if ( len && ( line[ len - 1 ] == '\\' ) ) {
 			len--;
 			rc = -EINVAL;
 			continue;