Fixed: 'dereference before null check' in stack.c
Fixed: 'dereference before null check' in stack.c

The pointer `theStack` is dereferenced before the if statement that
checks whether `theStack` is null. This is fixed by placing the variable
declarations in the if statement.

ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz,ellson/graphviz