service: advapi: Code cleaning

This patch fixes various things, from Coverity code defects to conversion
warnings :

• CID 715948 (logically dead code @ services/rpcserver.c)
• try to fix CID 716332/3 (resource leaks) by rewriting the ScmReadString
  function (@ services/config.c)
• zero out the freshly allocated memory (@ services)
• try to fix CID 716126/7/8 (untrusted value as argument @
  advapi32/services/sctrl.c)

Fix also some "size_t to DWORD" warnings on x64 build (@
advapi32/services/scm.c)