go/types: fix crash following misuse of [...]T in composite literal

The type-checker currently crashes when checking code such as:

	_ = map[string][...]int{"": {1, 2, 3}}

In this case, the type checker reports an error for map[string][...]int,
then proceeds to type-check the values of the map literal using a hint
type of [...]int. When type-checking the inner composite (array) literal,
the length of the open array type is computed from the elements,
then the array type is recorded, but the literal has no explicit type
syntax against which to record the type, so this code causes the
type-checker to panic. Add a nil check before calling
check.recordTypeAndValue to avoid that.

Updates #22467

Change-Id: Ic4453ba485b7b88ede2a89f209365eda9e032abc
Reviewed-on: https://go-review.googlesource.com/132355


Reviewed-by: Alan Donovan <adonovan@google.com>