[Coverity CID :203623]Memory - illegal accesses in /tests/subsys/settings/fcb_init/src/settings_test_fcb_init.c

[aasthagr]

Static code scan issues seen in File: /tests/subsys/settings/fcb_init/src/settings_test_fcb_init.c
Category: Memory - illegal accesses
Function: test_prepare_storage
Component: Tests
CID: 203623
Please fix or provide comments to square it off in coverity in the link: https://scan9.coverity.com/reports.htm#v32951/p12996

[aasthagr]

*** CID 203623:  Memory - illegal accesses  (NO_EFFECT)
/tests/subsys/settings/fcb_init/src/settings_test_fcb_init.c: 109 in test_prepare_storage()
103     
104     		dev = flash_area_get_device(fa);
105     
106     		err = flash_write_protection_set(dev, false);
107     		zassert_true(err == 0, "can't unprotect flash");
108     
>>>     CID 203623:  Memory - illegal accesses  (NO_EFFECT)
>>>     Argument "-256" in "memset" loses precision in "memset(new_val, -256, 4U)".
109     		(void)memset(new_val, ~ERASED_VAL, DT_FLASH_WRITE_BLOCK_SIZE);
110     		err = flash_write(dev, (off_t)&prepared_mark, &new_val,
111     				  sizeof(new_val));
112     		zassert_true(err == 0, "can't write prepared_mark");
113     	}
114     #else