Fix fgetrune on systems where char is unsigned by default (ARM)

Store the result in an int and do the comparison.  This is always
safe without using strange constructs like "signed char".

wc(1) would go into an infinite loop when executed on an ARM
system.

libutf/fgetrune.c

@@ -10,11 +10,13 @@ int
 fgetrune(Rune *r, FILE *fp)
 {
 	char buf[UTFmax];
-	int i;
+	int  i = 0, c;
 
-	for (i = 0; i < UTFmax && (buf[i] = fgetc(fp)) != EOF && ++i ;)
+	while (i < UTFmax && (c = fgetc(fp)) != EOF) {
+		buf[i++] = c;
 		if (charntorune(r, buf, i) > 0)
 			break;
+	}
 	if (ferror(fp))
 		return -1;