pax: fix signed-char left-shift UB in base64 encoder

Fix base64 shifts by casting input bytes to unsigned.

libarchive/archive_write_set_format_pax.c

@@ -2005,7 +2005,7 @@ base64_encode(const char *s, size_t len)
 	      'e','f','g','h','i','j','k','l','m','n','o','p','q','r','s',
 	      't','u','v','w','x','y','z','0','1','2','3','4','5','6','7',
 	      '8','9','+','/' };
-	int v;
+	uint32_t v;
 	char *d, *out;
 
 	/* 3 bytes becomes 4 chars, but round up and allow for trailing NUL */
@@ -2016,9 +2016,9 @@ base64_encode(const char *s, size_t len)
 
 	/* Convert each group of 3 bytes into 4 characters. */
 	while (len >= 3) {
-		v = (((int)s[0] << 16) & 0xff0000)
-		    | (((int)s[1] << 8) & 0xff00)
-		    | (((int)s[2]) & 0x00ff);
+		v = ((uint32_t)(unsigned char)s[0] << 16)
+		    | ((uint32_t)(unsigned char)s[1] << 8)
+		    | (uint32_t)(unsigned char)s[2];
 		s += 3;
 		len -= 3;
 		*d++ = digits[(v >> 18) & 0x3f];
@@ -2030,13 +2030,13 @@ base64_encode(const char *s, size_t len)
 	switch (len) {
 	case 0: break;
 	case 1:
-		v = (((int)s[0] << 16) & 0xff0000);
+		v = ((uint32_t)(unsigned char)s[0] << 16);
 		*d++ = digits[(v >> 18) & 0x3f];
 		*d++ = digits[(v >> 12) & 0x3f];
 		break;
 	case 2:
-		v = (((int)s[0] << 16) & 0xff0000)
-		    | (((int)s[1] << 8) & 0xff00);
+		v = ((uint32_t)(unsigned char)s[0] << 16)
+		    | ((uint32_t)(unsigned char)s[1] << 8);
 		*d++ = digits[(v >> 18) & 0x3f];
 		*d++ = digits[(v >> 12) & 0x3f];
 		*d++ = digits[(v >> 6) & 0x3f];