avcodec/flacdsp: Avoid undefined operations in non debug builds

This fixes ubsan warnings in non debug builds by using unsigned operations

in debug builds the correct signed operations are retained so that overflows
(which should not occur in valid files and may indicate problems in the DSP code
or decoder) can be detected.

Alternatively they can be changed to unsigned unconditionally, then its
not possible though to detect overflows easily if someone wants to test
the DSP code for overflows.

The 2nd alternative would be to leave the code as it is and accept that
there are undefined operations in the DSP code and that ubsan output is
full of them in some cases.

Similar changes would be needed in some other DSP routines

Suggested-by: Matt Wolenetz <wolenetz@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
