fmt_utf8 failed on ARM where char == unsigned char (Friedrich Beckmann)

fmt/fmt_utf8.c

@@ -16,7 +16,7 @@ size_t fmt_utf8(char *dest,uint32_t n) {
 	// width is undefined. That can't happen because both n>0x7f and
 	// i>=n would have to be true and we initialize i as 0x3f and
 	// with each iteration j is incremented. It's a false positive.
-	*dest++=(char)(((char)0xc0 >> (j-1)) | (char)(n >> k));
+	*dest++=(char)(((signed char)0xc0 >> (j-1)) | (char)(n >> k));
 	while (k) {
 	  *dest++=(char)(0x80 | ((n >> (k-6)) & 0x3f));
 	  k-=6;