libgcc: fix a warning calling find_fde_tail

The third parameter of find_fde_tail is an _Unwind_Ptr (which is an
integer type instead of a pointer), but we are passing NULL to it.  This
causes a -Wint-conversion warning.

libgcc/

	* unwind-dw2-fde-dip.c (_Unwind_Find_FDE): Call find_fde_tail
	with 0 instead of NULL.