target-ppc: Fix compiler warning

target-ppc: Fix compiler warning

gcc reports a warning which is usually wrong:

target-ppc/dfp_helper.c: In function ‘dfp_get_digit’:
target-ppc/dfp_helper.c:417:1: warning:
 control reaches end of non-void function [-Wreturn-type]

The compiler shows the warning if assert is not marked with the noreturn
attribute or if the code is compiled with -DNDEBUG.

Using g_assert_not_reached better documents the intention and does not
have these problems.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>