gdb/dwarf: remove nullptr prop check in dwarf2_evaluate_property

gdb/dwarf: remove nullptr prop check in dwarf2_evaluate_property

This nullptr is pointless most of the time: almost all callers either
check that the prop is not nullptr or they pass the address of a local
variable, which is necessarily not nullptr.  I propose to remove it.  I
found only one call site where I needed to add a check.

Change-Id: Iada84d529b8edc6b20486f3539c675dad6b364f5
Approved-By: Tom Tromey <tom@tromey.com>