Avoid undefined behavior in ada_operator_length

Avoid undefined behavior in ada_operator_length

-fsanitize=undefined pointed out this error:

    runtime error: load of value 2887952, which is not a valid value for type 'exp_opcode'

This happens in gdb.ada/complete.exp when processing "complete p
my_glob".  This does not parse, so the Ada parser throws an exception;
but then the code in parse_exp_in_context_1 accepts the expression
anyway.  However, as no elements have been written to the expression,
undefined behavior results.

The fix is to notice this case in parse_exp_in_context_1.  This patch
also adds an assertion to prefixify_expression to enforce this
pre-existing constraint.

gdb/ChangeLog
2018-10-03  Tom Tromey  <tom@tromey.com>

	* parse.c (prefixify_expression): Add assert.
	(parse_exp_in_context_1): Throw exception if the expression is
	empty.