btrfs-progs: initialize formatter context properly

In cases where the compiler does not initialize the formatter context to
all zeros, there could be garbage values left on the depth 0 that is not
explicitly initialized. This could lead to mistakenly printing a ","
separator before the last closing "}", like

{
  "__header": {
    "version": "1"
  },
}

Signed-off-by: David Sterba <dsterba@suse.com>

common/format-output.c

@@ -98,6 +98,7 @@ static void fmt_separator(struct format_ctx *fctx)
 void fmt_start(struct format_ctx *fctx, const struct rowspec *spec, int width,
 		int indent)
 {
+	memset(fctx, 0, sizeof(*fctx));
 	fctx->width = width;
 	fctx->indent = indent;
 	fctx->rowspec = spec;