Prevent segfault due to division by zero in music.get_pos (pygame-com…

…munity#2426)

music.get_pos now returns -1 on failure

docs/reST/ref/music.rst

@@ -217,6 +217,8 @@ MP3 in most cases.
    The returned time only represents how long the music has been playing; it
    does not take into account any starting position offsets.
 
+   Returns -1 if ``get_pos`` failed due to music not playing.
+
    .. ## pygame.mixer.music.get_pos ##
 
 .. function:: queue

src_c/music.c

@@ -252,12 +252,13 @@ music_get_pos(PyObject *self, PyObject *_null)
 
     MIXER_INIT_CHECK();
 
-    if (music_pos_time < 0)
+    Uint16 intermediate_step = (music_format & 0xff) >> 3;
+    long denominator = music_channels * music_frequency * intermediate_step;
+    if (music_pos_time < 0 || denominator == 0) {
         return PyLong_FromLong(-1);
+    }
 
-    ticks = (long)(1000 * music_pos /
-                   (music_channels * music_frequency *
-                    ((music_format & 0xff) >> 3)));
+    ticks = (long)(1000 * music_pos / denominator);
     if (!Mix_PausedMusic())
         ticks += PG_GetTicks() - music_pos_time;