mood.c: Silence gcc warning.

The newly released gcc-4.7.0 complains about "m" being used
uninitialized in change_current_mood(). gcc is wrong, but this is
not obvious to see: In change_current_mood(), "m" is only going to
be used if load_mood() returns non-negative. This happens only on
success, when load_mood() returns 1. In this case "m" was previously
set to mlpd.m which was initialized to NULL at the top of load_mood()
and later set to the newly allocated mood structure.

This patch makes it easier for gcc by initializing "m" to NULL
upfront. This causes the warning to go away.