Assert non-zero ptr when comparing mrb_value

[pulsejet]

For some reason, mrb_cptr gives 0 in certain cases (floats) on a wasm target, while the same script produces correct pointer values on x86.

When both of them are 0, the write_context assumes one is a link to the other and produces bad output. Checking one of them is 0 is good enough to prevent this.

The following script gives output as

class Class_B
  attr_reader   :z
  def initialize
    @z = 100.0
    @s = 235.0
  end
end

$b = Class_B.new
puts $b.z
puts Marshal.load(Marshal.dump($b)).z

On x86

100
100

Compiled with emscripten (wasm)

100
235

The value of z is stored as a link in the dump, as mrb_cptr(s) and mrb_cptr(z) are both 0, and both values are floats. I tried to figure out the reason for this behavior, but to no avail.