libswresample: Avoid needlessly large on-stack array.

We only actually need to use a tiny part of it.
Unfortunately we seem to have no real test coverage on
the code, so this is a bit risky.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libswresample/rematrix.c

@@ -56,6 +56,7 @@
 #define TOP_BACK_LEFT          15
 #define TOP_BACK_CENTER        16
 #define TOP_BACK_RIGHT         17
+#define NUM_NAMED_CHANNELS     18
 
 int swr_set_matrix(struct SwrContext *s, const double *matrix, int stride)
 {
@@ -112,7 +113,7 @@ static int sane_layout(int64_t layout){
 av_cold static int auto_matrix(SwrContext *s)
 {
     int i, j, out_i;
-    double matrix[64][64]={{0}};
+    double matrix[NUM_NAMED_CHANNELS][NUM_NAMED_CHANNELS]={{0}};
     int64_t unaccounted, in_ch_layout, out_ch_layout;
     double maxcoef=0;
     char buf[128];
@@ -145,7 +146,7 @@ av_cold static int auto_matrix(SwrContext *s)
     }
 
     memset(s->matrix, 0, sizeof(s->matrix));
-    for(i=0; i<64; i++){
+    for(i=0; i<FF_ARRAY_ELEMS(matrix); i++){
         if(in_ch_layout & out_ch_layout & (1ULL<<i))
             matrix[i][i]= 1.0;
     }
@@ -299,9 +300,12 @@ av_cold static int auto_matrix(SwrContext *s)
         double sum=0;
         int in_i=0;
         for(j=0; j<64; j++){
-            s->matrix[out_i][in_i]= matrix[i][j];
-            if(matrix[i][j]){
-                sum += fabs(matrix[i][j]);
+            if (i < FF_ARRAY_ELEMS(matrix) && j < FF_ARRAY_ELEMS(matrix[0]))
+                s->matrix[out_i][in_i]= matrix[i][j];
+            else
+                s->matrix[out_i][in_i]= i == j && (in_ch_layout & out_ch_layout & (1ULL<<i));
+            if(s->matrix[out_i][in_i]){
+                sum += fabs(s->matrix[out_i][in_i]);
             }
             if(in_ch_layout & (1ULL<<j))
                 in_i++;