dotgen: squash -Wsign-conversion warning in 'doDot'

dotgen: squash -Wsign-conversion warning in 'doDot'

The compiler says:

  dotinit.c: In function 'doDot':
  dotinit.c:454:24: warning: conversion to 'unsigned int' from 'int' may change
    the sign of the result [-Wsign-conversion]
  454 |         pinfo.margin = Pack;
      |                        ^~~~

But from the surrounding code and the implementation of `getPack`, we can
statically determine that `Pack` is non-negative at this point.