interp: fix some buggy localValue handling

Bug:

 1. fn.locals[v.value] returns 0 (the default value) if v.value is not
    part of the fn.locals map.
 2. locals[fn.locals[v.value]] then returns the first local value, which
    is usually non-nil
 3. This incorrect value is then used as the operand value.

The manifestation of this convoluted bug was
#2842. It didn't occur more
often probably because it only seems to happen in practice with inline
assembly.

Fixes #2842

Author: aykevl

interp/interpreter.go

@@ -81,12 +81,28 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 		if inst.opcode != llvm.PHI {
 			for _, v := range inst.operands {
 				if v, ok := v.(localValue); ok {
-					if localVal := locals[fn.locals[v.value]]; localVal == nil {
+					index, ok := fn.locals[v.value]
+					if !ok {
+						// This is a localValue that is not local to the
+						// function. An example would be an inline assembly call
+						// operand.
+						isRuntimeInst = true
+						break
+					}
+					localVal := locals[index]
+					if localVal == nil {
+						// Trying to read a function-local value before it is
+						// set.
 						return nil, mem, r.errorAt(inst, errors.New("interp: local not defined"))
 					} else {
 						operands = append(operands, localVal)
 						if _, ok := localVal.(localValue); ok {
+							// The function-local value is still just a
+							// localValue (which can't be interpreted at compile
+							// time). Not sure whether this ever happens in
+							// practice.
 							isRuntimeInst = true
+							break
 						}
 						continue
 					}