Icorrect identify info for calculated format "%@"

[SvyatoslavPozhydaev]

If use calculated format for indentify, them return wrong result. For example for PNG image 750x750 and with alpha chanel, always returned size width and height equal zero 0x0+750+750.

I assume this is from the -ping option: https://github.com/aheckmann/gm/blob/master/lib/getters.js#L271

If it is removed, then the correct result is returned.

[huxleyElateral]

also having the same issue https://github.com/aheckmann/gm/issues/861