ffasm.cpp overflow warnings to fix

/home/peter/Projects/ZeldaClassic/src/ffasm.cpp: In function int set_argument(char*, ffscript**, int, int):
/home/peter/Projects/ZeldaClassic/src/ffasm.cpp:2137:39: warning: %s directive writing up to 63 bytes into a region of size 20 [-Wformat-overflow=]
                 else sprintf(tempvar, "%s%d", variable_list[i].name, j);
                                       ^~~~~~
/home/peter/Projects/ZeldaClassic/src/ffasm.cpp:2137:29: note: sprintf output between 2 and 75 bytes into a destination of size 20
                 else sprintf(tempvar, "%s%d", variable_list[i].name, j);
                      ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/peter/Projects/ZeldaClassic/src/ffasm.cpp:2136:38: warning: %s directive writing up to 63 bytes into a region of size 20 [-Wformat-overflow=]
                     sprintf(tempvar, "%s%d", variable_list[i].name, j+1);
                                      ^~~~~~
/home/peter/Projects/ZeldaClassic/src/ffasm.cpp:2136:28: note: sprintf output between 2 and 75 bytes into a destination of size 20
                     sprintf(tempvar, "%s%d", variable_list[i].name, j+1);