Fix buffer overflow, detected by kASan.

[    1.044878] kASan: Unauthorized Access In 0xffffffff804ec7e2: Addr 0xffffffff818a51e4 [2 bytes, read]
[    1.044878] #0 0xffffffff804ec7e2 in mskc_probe <netbsd>
[    1.044878] #1 0xffffffff80e92a77 in mapply <netbsd>
[    1.044878] #2 0xffffffff80e92e5f in config_search_loc <netbsd>
[    1.044878] #3 0xffffffff80e93fb5 in config_found_sm_loc <netbsd>
[    1.044878] #4 0xffffffff802ca9ea in pci_probe_device <netbsd>
[    1.044878] #5 0xffffffff802cad97 in pci_enumerate_bus <netbsd>
[    1.044878] #6 0xffffffff802caf00 in pcirescan <netbsd>
[    1.044878] #7 0xffffffff802cb1ee in pciattach <netbsd>
[    1.044878] #8 0xffffffff80e93e5b in config_attach_loc <netbsd>
[    1.044878] #9 0xffffffff80e93fce in config_found_sm_loc <netbsd>
[    1.044878] #10 0xffffffff80271212 in mp_pci_scan <netbsd>
[    1.044878] #11 0xffffffff8022d9ee in mainbus_attach <netbsd>
[    1.044878] #12 0xffffffff80e93e5b in config_attach_loc <netbsd>
[    1.044878] #13 0xffffffff8021e38b in cpu_configure <netbsd>
[    1.044878] #14 0xffffffff814a7068 in main <netbsd>

(maxv)