slight optimization for sending 0 bytes from a file

(also fixes a FreeBSD-current sendfile breakage)

io/iob_addfile.c

@@ -1,7 +1,9 @@
 #include "iob_internal.h"
 
 int iob_addfile(io_batch* b,int64 fd,uint64 off,uint64 n) {
-  iob_entry* e=array_allocate(&b->b,sizeof(iob_entry),
+  iob_entry* e;
+  if (n==0) return 1;
+  e=array_allocate(&b->b,sizeof(iob_entry),
 			     array_length(&b->b,sizeof(iob_entry)));
   if (!e) return 0;
   e->type=FROMFILE;

io/iob_addfile_close.c

@@ -1,8 +1,10 @@
 #include "iob_internal.h"
 
 int iob_addfile_close(io_batch* b,int64 fd,uint64 off,uint64 n) {
-  iob_entry* e=array_allocate(&b->b,sizeof(iob_entry),
-			     array_length(&b->b,sizeof(iob_entry)));
+  iob_entry* e;
+  if (n==0) return 1;
+  e=array_allocate(&b->b,sizeof(iob_entry),
+		   array_length(&b->b,sizeof(iob_entry)));
   if (!e) return 0;
   e->type=FROMFILE_CLOSE;
   e->fd=fd;