Fix uninitialised memory acccess in COFF bfd backend
    
    Backported from master.
    
    2018-05-15  Christophe Guillon  <christophe.guillon@st.com>
    
    	* coffcode.h (coff_bigobj_swap_aux_in): Make sure that all fields
    	of the aux structure are initialised.
    
    Change-Id: I81be255ac6611afbe00995fac550e98e6a07e5df