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.

gdb/parse.c

@@ -792,6 +792,7 @@ copy_name (struct stoken token)
 int
 prefixify_expression (struct expression *expr)
 {
+  gdb_assert (expr->nelts > 0);
   int len = sizeof (struct expression) + EXP_ELEM_TO_BYTES (expr->nelts);
   struct expression *temp;
   int inpos = expr->nelts, outpos = 0;
@@ -1205,7 +1206,10 @@ parse_exp_in_context_1 (const char **stringptr, CORE_ADDR pc,
     }
   CATCH (except, RETURN_MASK_ALL)
     {
-      if (! parse_completion)
+      /* If parsing for completion, allow this to succeed; but if no
+	 expression elements have been written, then there's nothing
+	 to do, so fail.  */
+      if (! parse_completion || ps.expout_ptr == 0)
 	throw_exception (except);
     }
   END_CATCH