Fix thinko coming from 000f3adf

pg_basebackup.c relies on the compression level to not be 0 to decide if
compression should be used, but 000f3adf missed the fact that the
default compression (Z_DEFAULT_COMPRESSION) is -1, which would be used
if specifying --gzip without --compress.

While on it, add some coverage for --gzip, as this is rather easy to
miss.

Reported-by: Christoph Berg
Discussion: https://postgr.es/m/YdhRDMLjabtXOnhY@msg.df7cb.de

src/bin/pg_basebackup/pg_basebackup.c

@@ -524,7 +524,7 @@ LogStreamerMain(logstreamer_param *param)
 													stream.do_sync);
 	else
 		stream.walmethod = CreateWalTarMethod(param->xlog,
-											  (compresslevel > 0) ?
+											  (compresslevel != 0) ?
 											  COMPRESSION_GZIP : COMPRESSION_NONE,
 											  compresslevel,
 											  stream.do_sync);