introduce hex2chr() for converting two hexadecimal digits to a character

Add and use a helper function that decodes the char value of two
hexadecimal digits.  It returns a negative number on error, avoids
running over the end of the given string and doesn't shift negative
values.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hex.c

@@ -39,16 +39,8 @@ int get_sha1_hex(const char *hex, unsigned char *sha1)
 {
 	int i;
 	for (i = 0; i < GIT_SHA1_RAWSZ; i++) {
-		unsigned int val;
-		/*
-		 * hex[1]=='\0' is caught when val is checked below,
-		 * but if hex[0] is NUL we have to avoid reading
-		 * past the end of the string:
-		 */
-		if (!hex[0])
-			return -1;
-		val = (hexval(hex[0]) << 4) | hexval(hex[1]);
-		if (val & ~0xff)
+		int val = hex2chr(hex);
+		if (val < 0)
 			return -1;
 		*sha1++ = val;
 		hex += 2;