Ensure that Base64 output always wraps lines in the same manner as legacy implementation

The legacy Base64 conversion code in mbstring automatically
wrapped the output to 72 columns, and the new code imitates
this behavior. Frankly, I'm not sure if this is a good idea
or not (people could easily manually wrap it if they want to),
but have stuck with this behavior for backwards compatibility.

However, fuzzing revealed one case where we were not wrapping
to 72 columns; if the input string is not a multiple of 3
characters, meaning that the output must be padded, and the
point where we must add the final (padded) output happens to
be just beyond 72 columns.

Author: alexdowad

ext/mbstring/libmbfl/filters/mbfilter_base64.c

@@ -337,6 +337,10 @@ static void mb_wchar_to_base64(uint32_t *in, size_t len, mb_convert_buf *buf, bo
 	}
 
 	if (end && bits) {
+		if (chars_output > 72) {
+			out = mb_convert_buf_add2(out, '\r', '\n');
+			chars_output = 0;
+		}
 		if (bits == 8) {
 			out = mb_convert_buf_add4(out, mbfl_base64_table[(cache >> 2) & 0x3F], mbfl_base64_table[(cache & 0x3) << 4], '=', '=');
 		} else {