linux-user: fix Coverity CID 1464101

target_mmap() can fail and return -1, but we don't check for that and
instead assume it's always valid.

Fixes: db2af69d ("linux-user: Add infrastructure for a signal trampoline page")
Cc: richard.henderson@linaro.org
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211121151711.331653-1-laurent@vivier.eu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>