Ensure libpq reports a suitable error message on unexpected socket EOF.

The EOF-detection logic in pqReadData was a bit confused about who should
set up the error message in case the kernel gives us read-ready-but-no-data
rather than ECONNRESET or some other explicit error condition.  Since the
whole point of this situation is that the lower-level functions don't know
there's anything wrong, pqReadData itself must set up the message.  But
keep the assumption that if an errno was reported, a message was set up at
lower levels.

Per bug #11712 from Marko Tiikkaja.  It's been like this for a very long
time, so back-patch to all supported branches.

src/interfaces/libpq/fe-misc.c

@@ -764,12 +764,8 @@ retry3:
 			/* ready for read */
 			break;
 		default:
-			printfPQExpBuffer(&conn->errorMessage,
+			/* we override pqReadReady's message with something more useful */
-							  libpq_gettext(
+			goto definitelyEOF;
-								"server closed the connection unexpectedly\n"
-				   "\tThis probably means the server terminated abnormally\n"
-							 "\tbefore or while processing the request.\n"));
-			goto definitelyFailed;
 	}
 
 	/*
@@ -808,9 +804,16 @@ retry4:
 
 	/*
 	 * OK, we are getting a zero read even though select() says ready. This
-	 * means the connection has been closed.  Cope.  Note that errorMessage
+	 * means the connection has been closed.  Cope.
-	 * has been set already.
 	 */
+definitelyEOF:
+	printfPQExpBuffer(&conn->errorMessage,
+					  libpq_gettext(
+								"server closed the connection unexpectedly\n"
+				   "\tThis probably means the server terminated abnormally\n"
+							 "\tbefore or while processing the request.\n"));
+
+	/* Come here if lower-level code already set a suitable errorMessage */
 definitelyFailed:
 	pqDropConnection(conn);
 	conn->status = CONNECTION_BAD;		/* No more connection to backend */