Input validation

Gifsicle through 1.94, if deployed in a way that allows untrusted input to affect Gif_Realloc calls, might allow a denial of service (memory consumption). NOTE: this has been disputed by multiple parties because the Gifsicle code is not commonly used for unattended operation in which new input arrives for a long-running process, does not ship with functionality to link it into another application as a library, and does not have realistic use cases in which an adversary controls the entire command line.