HP-UX libunwind uses the sign of error code to indicate whether the
operation was successful; however, LLVM always returns positive error
codes.

Normalise the sign of the error code so that it handles all flavours of
libunwind.
Signed-off-by: Jinoh Kang <jinoh.kang.kr@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>