Fix uninitialised reads in path parser.

I suspect the actual problem is that rsvg_parse_path_do_cmd doesn't always fill
in the implicit args as 0 correctly (e.g. for arcs), but this should have the
same effect.

From upstream (rejected): https://gitlab.gnome.org/GNOME/librsvg/-/issues/654

--- librsvg-2.40.21/rsvg-path.c	2018-10-01 23:50:22.000000000 +0100
+++ librsvg-2.40.21/rsvg-path.c	2021-11-27 22:14:53.134375540 +0000
@@ -746,6 +746,7 @@
     ctx.rp.point.y = 0.0;
     ctx.cmd = 0;
     ctx.param = 0;
+    memset(ctx.params, 0, sizeof(ctx.params));
 
     rsvg_parse_path_data (&ctx, path_str);