cmd: spawn: reject 0 as an invalid job ID

cmd: spawn: reject 0 as an invalid job ID

Job IDs are positive integers greater than 1. 0 is not a valid job ID,
therefore fix the comparison in do_wait().

Fixes Coverity defects:

*** CID 550296:  Control flow issues  (NO_EFFECT)
/cmd/spawn.c: 172 in do_wait()
166                     for (i = 0; i < CONFIG_CMD_SPAWN_NUM_JOBS; i++)
167                             if (job[i])
168                                     ret = wait_job(i);
169             } else {
170                     for (i = 1; i < argc; i++) {
171                             id = dectoul(argv[i], NULL);
>>>     CID 550296:  Control flow issues  (NO_EFFECT)
>>>     This less-than-zero comparison of an unsigned value is never true.
"id < 0UL".
172                             if (id < 0 || id >
CONFIG_CMD_SPAWN_NUM_JOBS)
173                                     return CMD_RET_USAGE;
174                             idx = (int)id - 1;
175                             ret = wait_job(idx);
176                     }
177             }

*** CID 550297:  Integer handling issues  (INTEGER_OVERFLOW)
/cmd/spawn.c: 174 in do_wait()
168                                     ret = wait_job(i);
169             } else {
170                     for (i = 1; i < argc; i++) {
171                             id = dectoul(argv[i], NULL);
172                             if (id < 0 || id >
CONFIG_CMD_SPAWN_NUM_JOBS)
173                                     return CMD_RET_USAGE;
>>>     CID 550297:  Integer handling issues  (INTEGER_OVERFLOW)
>>>     Expression "idx", where "(int)id - 1" is known to be equal to -1,
overflows the type of "idx", which is type "unsigned int".
174                             idx = (int)id - 1;
175                             ret = wait_job(idx);
176                     }
177             }

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
CC: Tom Rini <trini@konsulko.com>