raop-sink: Fix compiler warnings

There were three maybe-uninitialized warnings when building with Autotools (for some reason I don't see these with Meson):

modules/raop/raop-sink.c: In function ‘thread_func’:
modules/raop/raop-sink.c:543:16: warning: ‘intvl’ may be used uninitialized in this function [-Wmaybe-uninitialized]
             if (intvl < now + u->block_usec) {
                ^
In file included from ./pulsecore/macro.h:270,
                 from ./pulsecore/cpu-x86.h:25,
                 from ./pulsecore/cpu.h:23,
                 from ./pulsecore/core.h:26,
                 from modules/raop/raop-sink.c:48:
./pulsecore/log.h:129:28: warning: ‘check_timing_count’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 #define pa_log_warn(...)   pa_log_level_meta(PA_LOG_WARN,   __FILE__, __LINE__, __func__, __VA_ARGS__)
                            ^~~~~~~~~~~~~~~~~
modules/raop/raop-sink.c:404:14: note: ‘check_timing_count’ was declared here
     uint32_t check_timing_count;
              ^~~~~~~~~~~~~~~~~~
modules/raop/raop-sink.c:500:27: warning: ‘last_timing’ may be used uninitialized in this function [-Wmaybe-uninitialized]
                 pa_usec_t since = now - last_timing;
                           ^~~~~

I moved the intvl variable initialization out of the for loop, because it looked like the variable value is supposed to be remembered between the iterations. I don't know if the variable declaration (without initialization) in the beginning of the loop caused the compiler to touch the variable between iterations, probably not, but I'm pretty sure that's undefined behaviour.

Other than that maybe-undefined behaviour, these compiler warnings may be false positives, since the variables are initialized when u->first is true.

I initialized the three variables in to the same value as what is used when resetting them when u->first is true. I didn't test these changes, but they look safe to me.