codecgen: set init value of numToRead ("rr") to container Len.

Fixes #129

codec/gen-dec-array.go.tmpl

@@ -27,7 +27,8 @@ if {{var "l"}} == 0 {
 	}
 	{{ else }}	var {{var "rr"}}, {{var "rl"}} int {{/* // num2read, length of slice/array/chan */}}
 	var {{var "rt"}} bool {{/* truncated */}}
-	_, _, _ = {{var "rr"}}, {{var "rl"}}, {{var "rt"}}
+	_, _ = {{var "rl"}}, {{var "rt"}}
+	{{var "rr"}} = {{var "l"}} // len({{var "v"}})
 	if {{var "l"}} > cap({{var "v"}}) {
 		{{if isArray }}z.DecArrayCannotExpand(len({{var "v"}}), {{var "l"}})
 		{{ else }}{{if not .Immutable }}

codec/gen.generated.go

@@ -96,7 +96,8 @@ if {{var "l"}} == 0 {
 	}
 	{{ else }}	var {{var "rr"}}, {{var "rl"}} int {{/* // num2read, length of slice/array/chan */}}
 	var {{var "rt"}} bool {{/* truncated */}}
-	_, _, _ = {{var "rr"}}, {{var "rl"}}, {{var "rt"}}
+	_, _ = {{var "rl"}}, {{var "rt"}}
+	{{var "rr"}} = {{var "l"}} // len({{var "v"}})
 	if {{var "l"}} > cap({{var "v"}}) {
 		{{if isArray }}z.DecArrayCannotExpand(len({{var "v"}}), {{var "l"}})
 		{{ else }}{{if not .Immutable }}