Description:
------------
Hi, I found an infinity loop in GD GIF core parsing function. 

It is easy to trigger in web application if the web use GD as its image library. For example, It can be triggered by the following functions

* imagecreatefromgif
* imagecreatefromstring

This vulnerability will lead to a Denied of Service and exhausted the server resource.



There is a do-while in file `ext/gd/libgd/gd_gif_in.c` and function `LWZReadByte_`
```
do {
    sd->firstcode = sd->oldcode =
    GetCode(fd, &sd->scd, sd->code_size, FALSE, ZeroDataBlockP);
} while (sd->firstcode == sd->clear_code);
```
https://github.com/php/php-src/blob/c5767db441e4db2a1e07b5880129ad7ce0b25b6f/ext/gd/libgd/gd_gif_in.c#L460


The implementation of `GetCode` is in `GetCode_`
```
static int
GetCode_(gdIOCtx *fd, CODE_STATIC_DATA *scd, int code_size, int flag, int *ZeroDataBlockP)
{
    int           i, j, ret;
    unsigned char count;

    ... 

    if ((count = GetDataBlock(fd, &scd->buf[2], ZeroDataBlockP)) <= 0)
        scd->done = TRUE;

    ...
}
```
https://github.com/php/php-src/blob/c5767db441e4db2a1e07b5880129ad7ce0b25b6f/ext/gd/libgd/gd_gif_in.c#L376

As you can see, `GetDataBlock` will read the image data and return the length. If EOF, returned -1. But the variable `count` is `unsigned char`, will always be positive value. So the line `scd->done = TRUE` will never be executed.


I think the easiest patch is change `unsigned char` to `int`.



Test script:
---------------
$ curl https://gist.githubusercontent.com/orangetw/adb0e2519df267eb54d8b68027a91d4c/raw/7a7d6938f59dd89e9a9b7304d71f8f6640609479/poc.gif.xxd | xxd -r > poc.gif

$ php -r 'imagecreatefromgif("poc.gif");'
hang here...