Undefined symbols have a value of 0, so it makes no sense to return such
a symbol when performing a lookup by value. This occurs when unwinding
the stack after calling a NULL function pointer, and we confusingly
report the current function as uart_sab82532_class() on amd64.
Convert db_print_loc_and_inst() to only attempt disassembly if we
managed to find a symbol corresponding to the IP. Otherwise we may fault
and reenter the debugger.