Very long strings cause stack overflow

[PlanetEarthSoftware]

When processing strings DotParser recurses for every character in the lexer. This causes the stack to overflow on very long, (about 16k) strings. Dot itself only handles string up to about 16k in length. Attached is a .dot file that is processed by dot ok but causes a stack overflow when processed by DotParser. It would be ideal if DotParser could handle strings of any length as .dot files can be used to interchange data between non GraphVis applications that use long strings.
long-label-16k.dot.zip

long-label-16k.dot.svg.zip