interp: defer out-of-bounds loads to runtime instead of crashing

[iamrajiv]

Fixes #4214

When the interpreter encountered a load that was out of bounds of the object, it panicked with "interp: load out of bounds", crashing the compiler. This can happen for valid Go programs, for example when dereferencing the pointer returned by unsafe.SliceData on a zero-capacity slice, which points to a zero-sized object:

package main

import "unsafe"

var p = unsafe.SliceData([]int{})
var v = *p

func main() {}

Return nil for an out-of-bounds load, the same as for an external global, so the caller defers the load to runtime instead of crashing. This matches what regular Go does, where the load reads from the runtime zero-base at runtime.

[dgryski]

/cc @aykevl for interp

[jakebailey]

Based on my own (limited) experience in interp, this seems right to me? e.g. in my PR to limit interp iterations bailing out is the right way to get it to do something at runtime instead. The only reason this code should panic is for internal errors, not because the user code does something that will panic at runtime.