strparser: Propagate correct error code in strp_recv()

With m68k-linux-gnu-gcc-4.1:

    net/strparser/strparser.c: In function ‘strp_recv’:
    net/strparser/strparser.c:98: warning: ‘err’ may be used uninitialized in this function

Pass "len" (which is an error code when negative) instead of the
uninitialized "err" variable to fix this.

Fixes: 43a0c675 ("strparser: Stream parser for messages")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>