tcg/s390x: Return false on failure from patch_reloc

tcg/s390x: Return false on failure from patch_reloc

This does require an extra two checks within the slow paths
to replace the assert that we're moving.  Also add two checks
within existing functions that lacked any kind of assert for
out of range branch.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>