Silence -Wimplicit-fallthrough in sysv_shmem.c.

Commit 2f932f7 added code that elicits
a warning on buildfarm member flaviventris.  Back-patch to 9.4, like
that commit.

Reported by Andres Freund.

Discussion: https://postgr.es/m/20190404020057.galelv7by75ekqrh@alap3.anarazel.de

Author: nmisch

src/backend/port/sysv_shmem.c

@@ -672,6 +672,7 @@ PGSharedMemoryCreate(Size size, int port,
 								(unsigned long) shmid),
 						 errhint("Terminate any old server processes associated with data directory \"%s\".",
 								 DataDir)));
+				break;
 			case SHMSTATE_ENOENT:
 
 				/*
@@ -703,6 +704,7 @@ PGSharedMemoryCreate(Size size, int port,
 					dsm_cleanup_using_control_segment(oldhdr->dsm_control);
 				if (shmctl(shmid, IPC_RMID, NULL) < 0)
 					NextShmemSegID++;
+				break;
 		}
 
 		if (oldhdr && shmdt(oldhdr) < 0)