amem: fix integer overflow above 262143Hz

The variable 'rate' in aout_sys_t had 18bits, which was not enough to represent
values greater than 262143.

This patch also fixes an inconsistency in the number of supported channels.
According to source code, where channel mapping is configured, amem only
supports 8 channels.

Signed-off-by: Farid Hammane <farid.hammane@gmail.com>

modules/audio_output/amem.c

@@ -31,6 +31,7 @@ static int Open (vlc_object_t *);
 static void Close (vlc_object_t *);
 
 #define AMEM_SAMPLE_RATE_MAX 384000
+#define AMEM_CHAN_MAX 8
 
 vlc_module_begin ()
     set_shortname (N_("Audio memory"))
@@ -49,7 +50,7 @@ vlc_module_begin ()
         change_private()
     add_integer ("amem-channels", 2,
                  N_("Channels count"), N_("Channels count"), false)
-        change_integer_range (1, AOUT_CHAN_MAX)
+        change_integer_range (1, AMEM_CHAN_MAX)
         change_private()
 
 vlc_module_end ()
@@ -67,8 +68,8 @@ typedef struct
         };
         struct
         {
-             unsigned rate:18;
-             unsigned channels:14;
+             unsigned rate;
+             uint8_t channels;
         };
     };
     void (*play) (void *opaque, const void *data, unsigned count, int64_t pts);
@@ -229,7 +230,7 @@ static int Start (audio_output_t *aout, audio_sample_format_t *fmt)
 
     /* Ensure that format is supported */
     if (fmt->i_rate == 0 || fmt->i_rate > AMEM_SAMPLE_RATE_MAX
-     || channels == 0 || channels > AOUT_CHAN_MAX
+     || channels == 0 || channels > AMEM_CHAN_MAX
      || strcmp(format, "S16N") /* TODO: amem-format */)
     {
         msg_Err (aout, "format not supported: %s, %u channel(s), %u Hz",