Initialize 'imm' on opcodes/aarch64-opc.c:expand_fp_imm (and fix breakage on mingw)

Initialize 'imm' on opcodes/aarch64-opc.c:expand_fp_imm (and fix breakage on mingw)

Hi,

While compiling GDB using a mingw compiler from Fedora 26:

  ../gdb/configure --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 \
    --disable-binutils --disable-ld --disable-gold --disable-gas --disable-sim \
    --disable-gprof  --enable-targets=all

I stumbled upon a simple occurrence of -Werror=maybe-uninitialized:

  ../../gdb/opcodes/aarch64-opc.c: In function 'expand_fp_imm':
  ../../gdb/opcodes/aarch64-opc.c:2880:10: error: 'imm' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     return imm;
	    ^~~

It is the compiler's fault, because this function always assigns to
'imm' if the necessary conditions are met, and it calls "assert (0)"
otherwise, but I thought it'd be clearer to have 'imm' explicitly set
to zero anyway.

opcodes/ChangeLog:
2017-09-21  Sergio Durigan Junior  <sergiodj@redhat.com>

	* aarch64-opc.c (expand_fp_imm): Initialize 'imm'.