mixer.c: Fixed Clang's Code model warning

Because it thinks that `num_channels` may have the value less than 1 (even there is no actual code that may assign it), it reports about the possible null pointer dereference at the `mix_channel` array pointer when `numchans` is zero.

src/mixer.c

@@ -625,6 +625,9 @@ int MIXCALLCC Mix_AllocateChannels(int numchans)
  if (mix_channel_tmp || !numchans) {
  /* Apply the temporary pointer on success */
  mix_channel = mix_channel_tmp;
+ if (num_channels < 0) {
+ num_channels = 0;
+ }
  if (numchans > num_channels) {
  /* Initialize the new channels */
  for (i = num_channels; i < numchans; i++) {