xkbcomp: Don't falsely promise from ExprResolveLhs

Every user of ExprReturnLhs goes on to unconditionally dereference the
field return, which can be NULL if xkb_intern_atom fails. Return false
if this is the case, so we fail safely.

testcase: splice geometry data into interp

Signed-off-by: Daniel Stone <daniels@collabora.com>

src/xkbcomp/expr.c

@@ -42,7 +42,7 @@ ExprResolveLhs(struct xkb_context *ctx, const ExprDef *expr,
         *elem_rtrn = NULL;
         *field_rtrn = xkb_atom_text(ctx, expr->ident.ident);
         *index_rtrn = NULL;
-        return true;
+        return (*field_rtrn != NULL);
     case EXPR_FIELD_REF:
         *elem_rtrn = xkb_atom_text(ctx, expr->field_ref.element);
         *field_rtrn = xkb_atom_text(ctx, expr->field_ref.field);