Give a proper error message if initdb password file is empty.

Used to say just "could not read password from file "...": Success", which
isn't very informative.

Mats Erik Andersson. Backpatch to all supported versions.

src/bin/initdb/initdb.c

@@ -1663,8 +1663,12 @@ get_set_pwd(void)
 		}
 		if (!fgets(pwdbuf, sizeof(pwdbuf), pwf))
 		{
-			fprintf(stderr, _("%s: could not read password from file \"%s\": %s\n"),
-					progname, pwfilename, strerror(errno));
+			if (ferror(pwf))
+				fprintf(stderr, _("%s: could not read password from file \"%s\": %s\n"),
+						progname, pwfilename, strerror(errno));
+			else
+				fprintf(stderr, _("%s: password file \"%s\" is empty\n"),
+						progname, pwfilename);
 			exit_nicely();
 		}
 		fclose(pwf);