proc: do not panic if we can't satisfy a composite location

When a location expression requests a register check that we have as
many bytes in the register as requested and if we don't report the
error.

Updates #1416

pkg/proc/mem.go

@@ -2,6 +2,7 @@ package proc
 
 import (
 	"errors"
+	"fmt"
 
 	"github.com/derekparker/delve/pkg/dwarf/op"
 )
@@ -93,7 +94,7 @@ type compositeMemory struct {
 	data    []byte
 }
 
-func newCompositeMemory(mem MemoryReadWriter, regs op.DwarfRegisters, pieces []op.Piece) *compositeMemory {
+func newCompositeMemory(mem MemoryReadWriter, regs op.DwarfRegisters, pieces []op.Piece) (*compositeMemory, error) {
 	cmem := &compositeMemory{realmem: mem, regs: regs, pieces: pieces, data: []byte{}}
 	for _, piece := range pieces {
 		if piece.IsRegister {
@@ -102,6 +103,9 @@ func newCompositeMemory(mem MemoryReadWriter, regs op.DwarfRegisters, pieces []o
 			if sz == 0 && len(pieces) == 1 {
 				sz = len(reg)
 			}
+			if sz > len(reg) {
+				return nil, fmt.Errorf("could not read %d bytes from register %d (size: %d)", sz, piece.RegNum, len(reg))
+			}
 			cmem.data = append(cmem.data, reg[:sz]...)
 		} else {
 			buf := make([]byte, piece.Size)
@@ -109,7 +113,7 @@ func newCompositeMemory(mem MemoryReadWriter, regs op.DwarfRegisters, pieces []o
 			cmem.data = append(cmem.data, buf...)
 		}
 	}
-	return cmem
+	return cmem, nil
 }
 
 func (mem *compositeMemory) ReadMemory(data []byte, addr uintptr) (int, error) {

pkg/proc/variables.go

@@ -908,7 +908,10 @@ func (scope *EvalScope) extractVarInfoFromEntry(varEntry *dwarf.Entry) (*Variabl
 	mem := scope.Mem
 	if pieces != nil {
 		addr = fakeAddress
-		mem = newCompositeMemory(scope.Mem, scope.Regs, pieces)
+		mem, err = newCompositeMemory(scope.Mem, scope.Regs, pieces)
+		if mem == nil {
+			mem = scope.Mem
+		}
 	}
 
 	v := scope.newVariable(n, uintptr(addr), t, mem)