glsl: Reject C-style initializers with unknown types.

glsl: Reject C-style initializers with unknown types.

_mesa_ast_set_aggregate_type walks through declarations initialized with
C-style aggregate initializers and stops when it runs out of LHS
declarations or RHS expressions.

In the example

   vec4 v = {{{1, 2, 3, 4}}};

_mesa_ast_set_aggregate_type would not recurse into the subexpressions
(since vec4s do not contain types that can be initialized with an
aggregate initializer) to set their <constructor_type>s. Later in ::hir
we would dereference the NULL pointer and segfault.

If <constructor_type> is NULL in ::hir we know that the LHS and RHS
were unbalanced and the code is illegal.

Arrays, structs, and matrices were unaffected.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>