target/riscv-013: fix unchecked return code

target/riscv-013: fix unchecked return code

Scan-build complains about variable 'sbcs_orig' that can be used
not initialized.
	Logic error: Assigned value is garbage or undefined
	riscv-013.c:4468 Assigned value is garbage or undefined
This is caused by not checking the return value of the call
	riscv-013.c:4466 dmi_read(target, &sbcs_orig, DM_SBCS);
In fact when dmi_read() returns error, the variable 'sbcs_orig' is
not assigned.

Check the returned value.

Change-Id: Ia9032a0229aa243138f95f4e13f765726a4ceae9
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7205
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Jan Matyas <matyas@codasip.com>
Tested-by: jenkins