flush(): avoid memcpy(<dst>, NULL, 0)

During thread contention, it's possible for a thread to not call push()
even once, leaving gs_thread[me] unallocated. The resulting call to
memcpy() was noticed by a sanitizer during additional CRAN checks.

Author: aitap

src/forder.c

@@ -128,6 +128,7 @@ static void flush(void) {
   if (!retgrp) return;
   int me = omp_get_thread_num();
   int n = gs_thread_n[me];
+  if (!n) return;
   int newn = gs_n + n;
   if (gs_alloc < newn) {
     gs_alloc = (newn < nrow/3) ? (1+(newn*2)/4096)*4096 : nrow;