utf8.c: avoid char overflow

In ISO C, char constants must be in the range -128..127. Change the BOM
constants to char literals to avoid overflow.

Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

utf8.c

@@ -566,10 +566,10 @@ static int has_bom_prefix(const char *data, size_t len,
 	return data && bom && (len >= bom_len) && !memcmp(data, bom, bom_len);
 }
 
-static const char utf16_be_bom[] = {0xFE, 0xFF};
-static const char utf16_le_bom[] = {0xFF, 0xFE};
-static const char utf32_be_bom[] = {0x00, 0x00, 0xFE, 0xFF};
-static const char utf32_le_bom[] = {0xFF, 0xFE, 0x00, 0x00};
+static const char utf16_be_bom[] = {'\xFE', '\xFF'};
+static const char utf16_le_bom[] = {'\xFF', '\xFE'};
+static const char utf32_be_bom[] = {'\0', '\0', '\xFE', '\xFF'};
+static const char utf32_le_bom[] = {'\xFF', '\xFE', '\0', '\0'};
 
 int has_prohibited_utf_bom(const char *enc, const char *data, size_t len)
 {