glsl: handle error case with ast_post_inc, ast_post_dec

Return ir_rvalue::error_value with ast_post_inc, ast_post_dec if
parser error was emitted previously. This way process_array_size
won't see bogus IR generated like with commit 9c676a64.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98699


Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
(cherry picked from commit 03a5acec)