gdb/python: do not hold on gdb.Symtab object from gdb.Symtab_and_line

gdb/python: do not hold on gdb.Symtab object from gdb.Symtab_and_line

Previous commit changed symtab_to_symtab_object() so each time it is
called with particula struct symtab* it returns the same object.

Therefore there's no longer need to hold on symtab object (gdb.Symtab)
from struct sal_object in order to preserve identity of Symtab object
held in gdb.Symtab_and_line.symtab property. This in turn allowed for
some simplification in various functions.

While at it I changed a couple of NULLs to nullptrs.

Approved-By: Tom Tromey <tom@tromey.com>