Fix integer sanitizer in clearkey.

Calculating index sometimes causes an unsigned integer overflow. This
throws a runtime error on integer sanitized builds.

 runtime error: unsigned integer overflow: 0 - 1 cannot be represented
 in type 'unsigned int'

Since this gets implicitly converted to ssize_t anyhow, make the
conversion explicit before decrementing to avoid the overflow.

Bug: 30969751
Test: Compiles.
Change-Id: Ibd39c1ba0dc64673743672ffc70c22c09f8e828a