ml-proto's grow_memory traps on integer overflow

[sunfishcode]

The design repo says that grow_memory should return -1 on allocation failure and does not mention the possibility of trapping:
https://github.com/WebAssembly/design/blob/master/AstSemantics.md#resizing

ml-proto's implementation for grow_memory traps on overflow when converting from pages to bytes, or if the size exceeds an internal implementation limit and does not appear to have code for returning -1 on allocation failure:
https://github.com/WebAssembly/spec/blob/master/ml-proto/spec/eval.ml#L288
https://github.com/WebAssembly/spec/blob/master/ml-proto/spec/memory.ml#L71

(Moving this issue from WebAssembly/design#714, since this appears to be an ml-proto issue rather than a design issue.)

[rossberg]

Fixed.