potential null pointer dereference in read_xml_arch_file.cpp

[HackerFoo]

.../vtr-verilog-to-routing/libs/libarchfpga/src/read_xml_arch_file.cpp: In function ‘void XmlReadArch(const char*, bool, t_arch*, std::vector<t_physical_tile_type>&, std::vector<t_logical_block_type>&)’:
.../vtr-verilog-to-routing/libs/libarchfpga/src/read_xml_arch_file.cpp:4804:24: warning: potential null pointer dereference [-Wnull-dereference]
         if (tile_port->type != block_port->type
             ~~~~~~~~~~~^~~~
.../vtr-verilog-to-routing/libs/libarchfpga/src/read_xml_arch_file.cpp:4804:24: warning: potential null pointer dereference [-Wnull-dereference]

[litghost]

This change is part of the equivalent tiles change, which has a PR open here: #296