Handle illegal bytes properly when converting to '7bit' encoding

Previously, mbstring would silently drop illegal bytes when converting a
string to '7bit' encoding.

Author: alexdowad

ext/mbstring/libmbfl/filters/mbfilter_7bit.c

@@ -83,6 +83,8 @@ int mbfl_filt_conv_any_7bit(int c, mbfl_convert_filter *filter)
 {
 	if (c >= 0 && c < 0x80) {
 		CK((*filter->output_function)(c, filter->data));
+	} else {
+		mbfl_filt_conv_illegal_output(c, filter);
 	}
 	return c;
 }