Prior to this fix, functions stored a string representation of their
contents. Switch them to storing a parsed source reference and the
tnode of the contents. This is part of an effort to avoid reparsing
a function's contents every time it executes.
try_get_child() was taking the address of a reference; clang was thereby
assuming it could not be null and so was dropping the null check. Ensure
we do not dereference a null pointer.
Fixes#4678
