thumb.php: Fix undefined variable $width

Follows-up 5ca94d2d26e80bb2a4ea9339a3adc31baf186d3a.

In thumb.php an undefined variable ($width) was used, triggering
fatal errors while processing requests that were passing thumbnail
width with px (pixel) suffix.

Change-Id: I2dc4f4d2979f8225d2a6211ec50cdc79438539a9
Merge "thumb.php: Fix undefined variable $width"