GD plugin: fix: zero I/O contexts on creation

GD plugin: fix: zero I/O contexts on creation

The GD plugin was creating `gdIOCtx` objects on the stack with some
uninitialized members. At time of writing, the GD docs¹ claim this struct’s
layout is:

  typedef struct gdIOCtx
  {
    int (*getC)(gdIOCtxPtr);
    int (*getBuf)(gdIOCtxPtr, void *, int wanted);

    void (*putC)(gdIOCtxPtr, int);
    int (*putBuf)(gdIOCtxPtr, const void *, int wanted);

    // seek must return 1 on SUCCESS, 0 on FAILURE. Unlike fseek!
    int (*seek)(gdIOCtxPtr, const int);
    long (*tell)(gdIOCtxPtr);

    void (*gd_free)(gdIOCtxPtr);
  } gdIOCtx;

So Graphviz’ usage was leaving `getC`, `getBuf`, `seek`, and `gd_free`
uninitialized. This seems to work out OK; Graphviz’ usage of libgd apparently
does not involve any code paths that use these members. But this does not seem
to be an API guarantee. This change zeroes these members for future stability.

¹ https://libgd.github.io/manuals/2.3.3/files/gd_io-h.html#gdIOCtx