Fix rare corner case in ntspawn.c

There could be cases when libc/proc/describefds.c calls calloc(handlecount, ..) with handlecount = 0 to initialize 
what will later become opt_lpExplicitHandleList.
but calloc(0, ..) does not always return null so if the chosen conditional is a->opt_lpExplicitHandleList then UpdateProcThreadAttribute produces a Windows error.
The modification avoids this problem.

libc/calls/ntspawn.c

@@ -166,7 +166,7 @@ static textwindows int ntspawn2(struct NtSpawnArgs *a, struct SpawnBlock *sb) {
         alist, 0, kNtProcThreadAttributeParentProcess, &a->opt_hParentProcess,
         sizeof(a->opt_hParentProcess), 0, 0);
   }
-  if (ok && a->opt_lpExplicitHandleList) {
+  if (ok && a->dwExplicitHandleCount) {
     ok = UpdateProcThreadAttribute(
         alist, 0, kNtProcThreadAttributeHandleList, a->opt_lpExplicitHandleList,
         a->dwExplicitHandleCount * sizeof(*a->opt_lpExplicitHandleList), 0, 0);