pg_basebackup: Cleaner handling when compression is multiply specified.

Tushar Ahuja discovered that if you use both --compress and --gzip,
or --compress multiple times, the last instance of one of these
options doesn't in all cases overwrite the compression level set by
an earlier option. That's not a serious bug, but it also has nothing
to recommend it. Repair.

Discussion: http://postgr.es/m/CA+TgmoZfP=rsZB_9vDGfhuNgSu_M_09UWu8SjvsP65y_1pQFCg@mail.gmail.com

src/bin/pg_basebackup/pg_basebackup.c

@@ -966,6 +966,12 @@ parse_compress_options(char *src, WalCompressionMethod *methodres,
 	int			firstlen;
 	char	   *firstpart;
 
+	/*
+	 * clear 'levelres' so that if there are multiple compression options,
+	 * the last one fully overrides the earlier ones
+	 */
+	*levelres = 0;
+
 	/* check if the option is split in two */
 	sep = strchr(src, ':');