lib/proto: suppress "may be used uninitialized" in send_file_complete()

gcc emits the following warning:

> proto.c: In function ‘send_file_complete’:
> proto.c:437:10: error: ‘buf’ may be used uninitialized
> [-Werror=maybe-uninitialized]
>   437 |   return send_file_chunk (g, 0, buf, 0);
>       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In theory, passing the 1-byte array "buf", with indeterminate contents, to
xdr_bytes() ultimately, could be fine -- assuming xdr_bytes() never reads
the contents of the buffer, due to the buffer size being zero. However,
the xdr_bytes() manual does not seem to guarantee this (it also does not
explicitly permit passing a NULL buffer alongside size=0, which would be
even simpler for the caller).

In order to shut up the compiler, just zero-initialize the buffer --
that's simpler than adding diagnostics pragmas. The "maybe-uninitialized"
warning is otherwise very useful, so keep it globally enabled (per
WARN_CFLAGS / WERROR_CFLAGS).

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20211011223627.20856-3-lersek@redhat.com>
Acked-by: Richard W.M. Jones <rjones@redhat.com>

lib/proto.c

@@ -433,7 +433,7 @@ send_file_cancellation (guestfs_h *g)
 static int
 send_file_complete (guestfs_h *g)
 {
-  char buf[1];
+  char buf[1] = { '\0' };
   return send_file_chunk (g, 0, buf, 0);
 }