Fixed getting the value of a string variable whose address is in a register
When getting the length of the string, getting the address was rejected if the address was in a register.
Where is the length of the string and how to get it is described as:
DW_OP_deref, ... (see tdebuginfodwarf3.appenddef_string)
DW_OP_push_object_address can describe a register, not an address in memory.
Note: Apparently due to a similar problem, GDB crashes when getting the value of a string variable when using dwarf3. I.e. it is possible that the error is in FPC, but I'm not sure. This MR solves the FpDebug problem without changing the FPC.