pulse-server: fix potential use of dangling pointer

`getpwuid_r()` puts the strings pointed to from the returned
passwd struct into the specified buffer. Previously, that
buffer technically didn't live long enough to be usable
in the `snprintf()` call - although in practice this didn't
appear to be a problem. A particular version of GCC 11 generates
the same machine code for this function regardless whether
this patch is applied or not. Still, fix this by moving
the buffer to an outer scope.