get_sha1_hex(): do not read past a NUL character

Previously, get_sha1_hex() would read one character past the end of a
null-terminated string whose strlen was an even number less than 40.
Although the function correctly returned -1 in these cases, the extra
memory access might have been to uninitialized (or even, conceivably,
unallocated) memory.

Add a check to avoid reading past the end of a string.

This problem was discovered by Thomas Rast <trast@student.ethz.ch>
using valgrind.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hex.c

@@ -39,7 +39,15 @@ int get_sha1_hex(const char *hex, unsigned char *sha1)
 {
 	int i;
 	for (i = 0; i < 20; i++) {
-		unsigned int val = (hexval(hex[0]) << 4) | hexval(hex[1]);
+		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)
 			return -1;
 		*sha1++ = val;