Fix autovacuum launcher attachment to its DSA

The autovacuum launcher doesn't actually do anything with its DSA other
than creating it and attaching to it, but it's been observed that after
longjmp'ing to the standard error handling block (for example after
getting SIGINT) the autovacuum enters an infinite loop reporting that it
cannot attach to its DSA anymore (which is correct, because it's already
attached to it.)  Fix by only attempting to attach if not already
attached.

I introduced this bug together with BRIN autosummarization in
7526e102.

Reported-by: Yugo Nagata.
Author: Thomas Munro.  I added the comment to go with it.
Discussion: https://postgr.es/m/20170621211538.0c9eae73.nagata@sraoss.co.jp

src/backend/postmaster/autovacuum.c

@@ -612,7 +612,8 @@ AutoVacLauncherMain(int argc, char *argv[])
 
 	/*
 	 * Set up our DSA so that backends can install work-item requests.  It may
-	 * already exist as created by a previous launcher.
+	 * already exist as created by a previous launcher; and we may even be
+	 * already attached to it, if we're here after longjmp'ing above.
 	 */
 	if (!AutoVacuumShmem->av_dsa_handle)
 	{
@@ -626,7 +627,7 @@ AutoVacLauncherMain(int argc, char *argv[])
 		AutoVacuumShmem->av_workitems = InvalidDsaPointer;
 		LWLockRelease(AutovacuumLock);
 	}
-	else
+	else if (AutoVacuumDSA == NULL)
 	{
 		AutoVacuumDSA = dsa_attach(AutoVacuumShmem->av_dsa_handle);
 		dsa_pin_mapping(AutoVacuumDSA);