Spurious warning when compiling mandelbrot

[Wilfred]

sample_programs/mandelbrot.bf:20:29 warning: This multiply loop tried to access cell -16 (offset -36 from current cell 20)
<]<+<<<<<<<<<]>>>>>>>>>[>>>>[-<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<+>>>>>>>>>>>>>
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[Wilfred]

This (and probably #13) too are because we compile [-<++>] to MultiplyMove { changes: { -1: 2 } }. We cannot execute this multiply move if the current cell pointer is 0!

Both execution.rs and llvm.rs will need updating.

[Wilfred]

Fixed.