Check dup2() results in syslogger

Consistently check the dup2() call results throughout syslogger.c.
It's pretty unlikely that they'll error out, but if they do,
ereport(FATAL) instead of blissfully continuing on.

Spotted by the Coverity scanner.

src/backend/postmaster/syslogger.c

@@ -210,8 +210,14 @@ SysLoggerMain(int argc, char *argv[])
 		close(fileno(stderr));
 		if (fd != -1)
 		{
-			dup2(fd, fileno(stdout));
-			dup2(fd, fileno(stderr));
+			if (dup2(fd, fileno(stdout)) < 0)
+					ereport(FATAL,
+							(errcode_for_file_access(),
+							 errmsg("could not redirect stdout: %m")));
+			if (dup2(fd, fileno(stderr)) < 0)
+					ereport(FATAL,
+							(errcode_for_file_access(),
+							 errmsg("could not redirect stderr: %m")));
 			close(fd);
 		}
 	}