advapi32: don't test buffer length if buffer is null

[Dan Kegel]

Trivial bugfix which does nothing except fix following two valgrind warnings.

 Conditional jump or move depends on uninitialised value(s)
    at 0x4A75F26: GetServiceDisplayNameA (service.c:1925)
    by 0x496ACE0: test_get_displayname (service.c:418)
    by 0x496FA12: func_service (service.c:1246)
    by 0x4970289: run_test (test.h:414)
    by 0x4970555: main (test.h:463)
  Uninitialised value was created by a stack allocation
    at 0x496AC96: test_get_displayname (service.c:404)
 Conditional jump or move depends on uninitialised value(s)
    at 0x4A75A03: GetServiceKeyNameA (service.c:1814)
    by 0x496C1A0: test_get_servicekeyname (service.c:645)
    by 0x496FA17: func_service (service.c:1247)
    by 0x4970289: run_test (test.h:414)
    by 0x4970555: main (test.h:463)
  Uninitialised value was created by a stack allocation
    at 0x496C156: test_get_servicekeyname (service.c:631)
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