Add nil check in code-gen converter for binary

[chengs041021]

The problem:

Converter doesn't check nil for binary type. So when it tries to convert binary to a nested field, for example out.NestedTwo.Two, it can cause NPEs. Without this fix, it can generate code like the following, while out.NestedTwo can be nil.

out.NestedTwo.Two = []byte(in.Two)

The fix:

Add nil check for binary type.

Testing:

Added test, failed and got:

out.One = (*string)(in.One)
out.NestedTwo.NestedNestedTwo.Two = []byte(in.Two)

Added code, test passed and got

out.One = (*string)(in.One)
if out.NestedTwo == nil {
out.NestedTwo = &structs.NestedBar{}
}
if out.NestedTwo.NestedNestedTwo == nil {
out.NestedTwo.NestedNestedTwo = &structs.NestedNestedBar{}
}
out.NestedTwo.NestedNestedTwo.Two = []byte(in.Two)

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Coverage increased (+0.005%) to 69.829% when pulling aa38399 on sc-binary-check-nil into 993a605 on master.

[abhishekparwal]

Otherwise it generates code like
, where it will always NPE at L213 because out.Request is always empty.

To verify, I:
Added test, failed and got:

out.One = (*string)(in.One)
out.NestedTwo.NestedNestedTwo.Two = []byte(in.Two)

Added code, test passed and got

out.One = (*string)(in.One)
if out.NestedTwo == nil {
out.NestedTwo = &structs.NestedBar{}
}
if out.NestedTwo.NestedNestedTwo == nil {
out.NestedTwo.NestedNestedTwo = &structs.NestedNestedBar{}
}
out.NestedTwo.NestedNestedTwo.Two = []byte(in.Two)

removed proprietary link, please describe the problem in a general way and not using specific proprietary links.

[abhishekparwal]

make a change for nits and description of the bug. approving it.

[abhishekparwal]

nit: remove a todo above since its now handled.

[chengs041021]

Done.