Zero supplied stat buffers in functions that pretend to stat

Zero supplied stat buffers in functions that pretend to stat

GDB has five places where it pretends to stat for bfd_openr_iovec.
Four of these only set the incoming buffer's st_size, leaving the
other fields unchanged, which is to say very likely populated with
random values from the stack.  remote_bfd_iovec_stat was fixed in
0a93529c56714b1da3d7106d3e0300764f8bb81c; this commit fixes the
other four.

gdb/ChangeLog:

	* jit.c (mem_bfd_iovec_stat): Zero supplied buffer.
	* minidebug.c (lzma_stat): Likewise.
	* solib-spu.c (spu_bfd_iovec_stat): Likewise.
	* spu-linux-nat.c (spu_bfd_iovec_stat): Likewise.