wmadec: Fix two dead stores.

wmadec: Fix two dead stores.

wmadec_filter.c:772:3: warning: Value stored to 'mult1' is never read
                mult1 = mult;
                ^       ~~~~
wmadec_filter.c:1015:3: warning: Value stored to 'n' is never read
                n = pwd->block_len;
                ^   ~~~~~~~~~~~~~~

Also convert a statement of the form x = x * y to x *= y;