Allow Clang 9 To Build Master

[ozmorph]

404 introduced 4x compiler errors that prevents Clang 9 from building master.

ozmorph@home:~/covid-sim/build$ clang++-9 --version
clang version 9.0.1-12 
Target: x86_64-pc-linux-gnu
Thread model: posix

/home/ozmorph/covid-sim/src/Update.cpp:1356:33: error: ordered comparison between pointer and zero ('int *' and 'int')
        assert(Cells[cellIndex].latent >= 0);
               ~~~~~~~~~~~~~~~~~~~~~~~ ^  ~
/usr/include/assert.h:93:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
/home/ozmorph/covid-sim/src/Update.cpp:1357:35: error: ordered comparison between pointer and zero ('int *' and 'int')
        assert(Cells[cellIndex].infected >= 0);
               ~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~
/usr/include/assert.h:93:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
/home/ozmorph/covid-sim/src/Update.cpp:1368:33: error: ordered comparison between pointer and zero ('int *' and 'int')
        assert(Cells[cellIndex].latent >= 0);
               ~~~~~~~~~~~~~~~~~~~~~~~ ^  ~
/usr/include/assert.h:93:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
/home/ozmorph/covid-sim/src/Update.cpp:1380:35: error: ordered comparison between pointer and zero ('int *' and 'int')
        assert(Cells[cellIndex].infected >= 0);
               ~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~
/usr/include/assert.h:93:27: note: expanded from macro 'assert'
     (static_cast <bool> (expr)                                         \
                          ^~~~
2 warnings and 4 errors generated.

This patch fixes it.