use uninited variable in process_img()

[kcwu]

Input

00000000: 3c69 6d67 2073 7263 3d3e                 <img src=>

Please review process_img().
If <img> tag has no height attribute and USE_IMAGE is not defined. Variable i will be uninitialized.
However https://github.com/tats/w3m/blob/master/file.c#L3468 uses i.

 if (w > 0 && i > 0) {

This issue is probably not security related.

This is found by afl-fuzz.