pkg/proc: pad variable mem in extractVarInfoFromEntry

On 64 bit system, the byte size of th following struct is 16:
    type myStruct struct {
       a int
       b uint32
    }
But extractVarInfoFromEntry only allocates a mem of 12 bytes for it.
When calling method of this struct with the "call" command, it will
result in this error:
    write out of bounds

This patch extends the mem by adding padding bytes to the end of the
mem.

Fixes go-delve#3364.

_fixtures/fncall.go

@@ -199,6 +199,15 @@ func (i Issue2698) String() string {
 	return fmt.Sprintf("%d %d %d %d", i.a, i.b, i.c, i.d)
 }
 
+type Issue3364 struct {
+	a int
+	b uint32
+}
+
+func (i Issue3364) String() string {
+	return fmt.Sprintf("%d %d", i.a, i.b)
+}
+
 func main() {
 	one, two := 1, 2
 	intslice := []int{1, 2, 3}
@@ -222,6 +231,10 @@ func main() {
 		c: 3,
 		d: 4,
 	}
+	issue3364 := Issue3364{
+		a: 1,
+		b: 2,
+	}
 
 	fn2clos := makeclos(pa)
 	fn2glob := call1
@@ -241,5 +254,5 @@ func main() {
 	d.Method()
 	d.Base.Method()
 	x.CallMe()
-	fmt.Println(one, two, zero, call, call0, call2, callexit, callpanic, callbreak, callstacktrace, stringsJoin, intslice, stringslice, comma, a.VRcvr, a.PRcvr, pa, vable_a, vable_pa, pable_pa, fn2clos, fn2glob, fn2valmeth, fn2ptrmeth, fn2nil, ga, escapeArg, a2, square, intcallpanic, onetwothree, curriedAdd, getAStruct, getAStructPtr, getVRcvrableFromAStruct, getPRcvrableFromAStructPtr, getVRcvrableFromAStructPtr, pa2, noreturncall, str, d, x, x2.CallMe(5), longstrs, regabistacktest, regabistacktest2, issue2698.String(), regabistacktest3, rast3, floatsum, ref)
+	fmt.Println(one, two, zero, call, call0, call2, callexit, callpanic, callbreak, callstacktrace, stringsJoin, intslice, stringslice, comma, a.VRcvr, a.PRcvr, pa, vable_a, vable_pa, pable_pa, fn2clos, fn2glob, fn2valmeth, fn2ptrmeth, fn2nil, ga, escapeArg, a2, square, intcallpanic, onetwothree, curriedAdd, getAStruct, getAStructPtr, getVRcvrableFromAStruct, getPRcvrableFromAStructPtr, getVRcvrableFromAStructPtr, pa2, noreturncall, str, d, x, x2.CallMe(5), longstrs, regabistacktest, regabistacktest2, issue2698.String(), issue3364.String(), regabistacktest3, rast3, floatsum, ref)
 }

pkg/proc/variables.go

@@ -1207,6 +1207,11 @@ func extractVarInfoFromEntry(tgt *Target, bi *BinaryInfo, image *Image, regs op.
 			}
 		}
 		if cmem != nil {
+			paddingBytes := int(t.Common().ByteSize) - len(cmem.data)
+			if paddingBytes > 0 && paddingBytes < bi.Arch.ptrSize {
+				padding := make([]byte, paddingBytes)
+				cmem.data = append(cmem.data, padding...)
+			}
 			mem = cmem
 		}
 	}

pkg/proc/variables_test.go

@@ -1266,6 +1266,7 @@ func TestCallFunction(t *testing.T) {
 		{`regabistacktest("one", "two", "three", "four", "five", 4)`, []string{`:string:"onetwo"`, `:string:"twothree"`, `:string:"threefour"`, `:string:"fourfive"`, `:string:"fiveone"`, ":uint8:8"}, nil},
 		{`regabistacktest2(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)`, []string{":int:3", ":int:5", ":int:7", ":int:9", ":int:11", ":int:13", ":int:15", ":int:17", ":int:19", ":int:11"}, nil},
 		{`issue2698.String()`, []string{`:string:"1 2 3 4"`}, nil},
+		{`issue3364.String()`, []string{`:string:"1 2"`}, nil},
 		{`regabistacktest3(rast3, 5)`, []string{`:[10]string:[10]string ["onetwo","twothree","threefour","fourfive","fivesix","sixseven","sevenheight","heightnine","nineten","tenone"]`, ":uint8:15"}, nil},
 		{`floatsum(1, 2)`, []string{":float64:3"}, nil},
 	}