[Bug 41799] New: winspool.drv: race can cause WINSPOOL_LoadSystemPrinters to fail

https://bugs.winehq.org/show_bug.cgi?id=41799

Bug ID: 41799
           Summary: winspool.drv: race can cause
                    WINSPOOL_LoadSystemPrinters to fail
           Product: Wine
           Version: 1.9.23
          Hardware: x86
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: -unknown
          Assignee: [e-mail]
          Reporter: [e-mail]
      Distribution: ---

Created attachment 56232
  --> https://bugs.winehq.org/attachment.cgi?id=56232
winspool.drv: Clear last error when creating mutex

WINSPOOL_LoadSystemPrinters creates a unique mutex and then checks if
GetLastError is ERROR_ALREADY_EXISTS.  Running 1.9.23 with staging or
unpatched, and current -HEAD, I'm seeing a race where sometimes a stale error
is left and it returns ERROR_ALREADY_EXISTS due to another component.  This
causes printing to always fail on my machine, since CUPS_LoadPrinters is never
called and therefore no printers exist.

This one-line patch has allowed printing to work on my system and still guards
against multiple initialisations.