Only call isqrt32() with a positive argument

Fixes test_opus_projection failure under ubsan, due to clz(0).

celt/mathops.c

@@ -38,7 +38,8 @@
 #include "mathops.h"
 
 /*Compute floor(sqrt(_val)) with exact arithmetic.
-  This has been tested on all possible 32-bit inputs.*/
+  _val must be greater than 0.
+  This has been tested on all possible 32-bit inputs greater than 0.*/
 unsigned isqrt32(opus_uint32 _val){
   unsigned b;
   unsigned g;

src/opus_multistream_encoder.c

@@ -108,12 +108,14 @@ static int validate_ambisonics(int nb_channels, int *nb_streams, int *nb_coupled
    int acn_channels;
    int nondiegetic_channels;
 
+   if (nb_channels < 1 || nb_channels > 227)
+      return 0;
+
    order_plus_one = isqrt32(nb_channels);
    acn_channels = order_plus_one * order_plus_one;
    nondiegetic_channels = nb_channels - acn_channels;
 
-   if (order_plus_one < 1 || order_plus_one > 15 ||
-       (nondiegetic_channels != 0 && nondiegetic_channels != 2))
+   if (nondiegetic_channels != 0 && nondiegetic_channels != 2)
       return 0;
 
    if (nb_streams)

src/opus_projection_encoder.c

@@ -86,15 +86,17 @@ static int get_order_plus_one_from_channels(int channels, int *order_plus_one)
   /* Allowed numbers of channels:
    * (1 + n)^2 + 2j, for n = 0...14 and j = 0 or 1.
    */
+  if (channels < 1 || channels > 227)
+    return OPUS_BAD_ARG;
+
   order_plus_one_ = isqrt32(channels);
   acn_channels = order_plus_one_ * order_plus_one_;
   nondiegetic_channels = channels - acn_channels;
+  if (nondiegetic_channels != 0 && nondiegetic_channels != 2)
+    return OPUS_BAD_ARG;
+
   if (order_plus_one)
     *order_plus_one = order_plus_one_;
-
-  if (order_plus_one_ < 1 || order_plus_one_ > 15 ||
-      (nondiegetic_channels != 0 && nondiegetic_channels != 2))
-    return OPUS_BAD_ARG;
   return OPUS_OK;
 }