net: e1000e: fix dead code in e1000e_write_packet_to_guest

Because is_first is declared inside a loop, it is always true.  The store
is dead, and so is the "else" branch of "if (is_first)".  is_last is
okay though.

Reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/e1000e_core.c

@@ -1507,14 +1507,14 @@ e1000e_write_packet_to_guest(E1000ECore *core, struct NetRxPkt *pkt,
     const E1000E_RingInfo *rxi;
     size_t ps_hdr_len = 0;
     bool do_ps = e1000e_do_ps(core, pkt, &ps_hdr_len);
+    bool is_first = true;
 
     rxi = rxr->i;
 
     do {
         hwaddr ba[MAX_PS_BUFFERS];
         e1000e_ba_state bastate = { { 0 } };
         bool is_last = false;
-        bool is_first = true;
 
         desc_size = total_size - desc_offset;