util: Heap-allocate 256K zlib buffer

The disk cache code tries to allocate a 256 Kbyte buffer on the stack.
Since musl only gives 80 Kbyte of stack space per thread, this causes a
trap.

See https://wiki.musl-libc.org/functional-differences-from-glibc.html#Thread-stack-size

(In musl-1.1.21 the default stack size has increased to 128K)

[mattst88]: Original author unknown, but I think this is small enough
            that it is not copyrightable.
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

(cherry picked from commit fd7b7f14d8)

src/util/disk_cache.c

@@ -732,7 +732,7 @@ static size_t
 deflate_and_write_to_disk(const void *in_data, size_t in_data_size, int dest,
                           const char *filename)
 {
-   unsigned char out[BUFSIZE];
+   unsigned char *out;
 
    /* allocate deflate state */
    z_stream strm;
@@ -749,6 +749,11 @@ deflate_and_write_to_disk(const void *in_data, size_t in_data_size, int dest,
    /* compress until end of in_data */
    size_t compressed_size = 0;
    int flush;
+
+   out = malloc(BUFSIZE * sizeof(unsigned char));
+   if (out == NULL)
+      return 0;
+
    do {
       int remaining = in_data_size - BUFSIZE;
       flush = remaining > 0 ? Z_NO_FLUSH : Z_FINISH;
@@ -770,6 +775,7 @@ deflate_and_write_to_disk(const void *in_data, size_t in_data_size, int dest,
          ssize_t written = write_all(dest, out, have);
          if (written == -1) {
             (void)deflateEnd(&strm);
+            free(out);
             return 0;
          }
       } while (strm.avail_out == 0);
@@ -784,6 +790,7 @@ deflate_and_write_to_disk(const void *in_data, size_t in_data_size, int dest,
 
    /* clean up and return */
    (void)deflateEnd(&strm);
+   free(out);
    return compressed_size;
 }