binutils: backport patch to fix build with gcc-4.9

* backported from 2.23.2, fixes:
work/x86_64-linux/binutils-native/2.23.1-r3/binutils-2.23.1/gas/write.c:409:24:
  error: 'dummy.fr_type' may be used uninitialized in this function [-Werror=maybe-uninitialized]
   gas_assert (prev_frag->fr_type != 0);
                        ^
work/x86_64-linux/binutils-native/2.23.1-r3/binutils-2.23.1/gas/write.c:392:9: note: 'dummy' was declared here
   fragS dummy, *prev_frag = &dummy;
         ^

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>