Fix compiler warning

Fixes a compiler warning about uninitialized values that looks like it's coming from Agg, but which is actually matplotlib's fault ;)
I don't think so.  When the code returned is `agg::path_cmd_stop`, the x and y values are actually meaningless and should be ignored by the caller.  This fix is *only* to silence a compiler warning that it is theoretically possible to get a vertex where the x y values were never set (if it were a 0-length path), since the compiler doesn't understand the semantics of the vertex codes.