Avoid memory leak on error path

This isn't covered by the tests, but manually failing this allocation in
vgdb results in a leak otherwise:

268,096 (5,952 direct, 262,144 indirect) bytes in 1 blocks are definitely lost
in loss record 1,574 of 1,601
   at 0x48407B4: malloc (vg_replace_malloc.c:381)
   by 0x74ACD86: deflateInit2_ (in /lib/x86_64-linux-gnu/libz.so.1.2.13)
   by 0x90EA5232: init_stream (fwrite.c:576)
   by 0x90EA5EB0: fwriteMain (fwrite.c:806)
   by 0x90EA79EE: fwriteR (fwriteR.c:310)

Author: aitap

src/fwrite.c

@@ -823,6 +823,7 @@ void fwriteMain(fwriteMainArgs args)
     if (!zbuffPool) {
       // # nocov start
       free(buffPool);
+      deflateEnd(&strm);
       STOP(_("Unable to allocate %zu MiB * %d thread compressed buffers; '%d: %s'. Please read ?fwrite for nThread, buffMB and verbose options."),
            zbuffSize / MEGA, nth, errno, strerror(errno));
       // # nocov end