Don't move pointer before start of object in move_word_rel()

The old code could move the variable 'pointer' backwards one character before the start
of the string (then incremented it again before returning).  Formally this is undefined
behaviour, so fix it.

include/ircaux.h

@@ -134,7 +134,7 @@ char *	base64_encode	(const void *data, size_t size);
 #define EOS 32767
 char	*BX_strsearch			(register char *, char *, char *, int);
 char	*BX_move_to_abs_word	(const register char *, char **, int);
-char	*BX_move_word_rel		(const register char *, char **, int);
+char	*BX_move_word_rel		(const char *, char **, int);
 char	*BX_extract		(char *, int, int);
 char	*BX_extract2		(const char *, int, int);
 int	BX_wild_match		(const char *, const char *);