Fix crash in is_nocall_function

Fix crash in is_nocall_function

is_nocall_function anticipates only being called for a function or a
method.  However, PR gdb/29871 points out a situation where an unusual
expression -- but one that parses to a valid, if extremely weird,
function call -- breaks this assumption.

This patch changes is_nocall_function to remove this assert and
instead simply return 'false' in this case.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29871