Fix crash when an unpriv user builds an Ether() packet without dst set

When an Ether() / IP() packet is built with IP.dst set and Ether.dst
not set, Scapy tries to fix Ether.dst by sending an ARP request. This
fails when the user cannot send packets.

Before:

    >>> p = str(Ether()/IP(dst="1.2.3.4"))
    Traceback (most recent call last):
      File "<console>", line 1, in <module>
      File "/usr/local/lib/python2.7/dist-packages/scapy/packet.py", line 272, in __str__
        return self.build()
      [...]
      File "/usr/local/lib/python2.7/dist-packages/scapy/arch/linux.py", line 414, in __init__
        self.ins = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.htons(type))
      File "/usr/lib/python2.7/socket.py", line 187, in __init__
        _sock = _realsocket(family, type, proto)
    error: [Errno 1] Operation not permitted

After:

    >>> p = str(Ether()/IP(dst="1.2.3.4"))
    WARNING: Mac address to reach destination not found. Using broadcast.