gh-96821: Fix undefined behaviour in audioop.c

[matthiasgoergens]

Left-shifting negative numbers is undefined behaviour.

Fortunately, multiplication works just as well, is (implementation) defined behaviour, and gets compiled to the same machine code as before by optimizing compilers.

• Issue: Enable -fstrict-overflow #96821

[matthiasgoergens]

@kumaraditya303@mdickinson@erlend-aasland Would you please have a look? Thanks!

[kumaraditya303]

Sorry but I won't have time for this in the near future, Mark or Erlend can take a look.

[mdickinson]

The changes LGTM at least in principle; I haven't done much in the way of testing.

[mdickinson]

@matthiasgoergens Can we revert the changes at lines 173 and 262 of the original code? I'm fairly sure that those changes aren't necessary, because the quantity being shifted is always nonnegative.

[mdickinson]

Can we revert the changes at lines 173 and 262 of the original code?

To be clear, I'm happy with the rest of the changes as-is, so if @JelleZijlstra is also happy, I think we can merge after those two reversions.

[matthiasgoergens]

@mdickinson Thanks. I'll revert the lines you asked about.

[matthiasgoergens]

@mdickinson@JelleZijlstra I applied the suggested changes.

Thanks for having a look!