Use fixed size array in good_for_rle

OptimizeHuffmanForRle is only called from TryOptimizeHuffmanForRle
with fixed length values of ZOPFLI_NUM_LL and ZOPFLI_NUM_D. Rather
than fighting with compilers about warnings in the malloc call, use
a fixed size array the size of the larger of these.

Fixes google#150

src/zopfli/deflate.c

@@ -434,7 +434,7 @@ more efficiently. length contains the size of the histogram.
 void OptimizeHuffmanForRle(int length, size_t* counts) {
   int i, k, stride;
   size_t symbol, sum, limit;
-  int* good_for_rle;
+  int good_for_rle[ZOPFLI_NUM_LL];
 
   /* 1) We don't want to touch the trailing zeros. We may break the
   rules of the format by adding more data in the distance codes. */
@@ -449,7 +449,7 @@ void OptimizeHuffmanForRle(int length, size_t* counts) {
   }
   /* 2) Let's mark all population counts that already can be encoded
   with an rle code.*/
-  good_for_rle = (int*)malloc((unsigned)length * sizeof(int));
+  assert(length > 0 && length <= ZOPFLI_NUM_LL);
   for (i = 0; i < length; ++i) good_for_rle[i] = 0;
 
   /* Let's not spoil any of the existing good rle codes.
@@ -513,8 +513,6 @@ void OptimizeHuffmanForRle(int length, size_t* counts) {
       sum += counts[i];
     }
   }
-
-  free(good_for_rle);
 }
 
 /*