What does the following error mean?
==40399== Invalid write of size 4
==40399==    at 0x1092C2: split_array (split_array.c:25)
==40399==    by 0x1093B5: main (split_array.c:59)
==40399==  Address 0x4a73134 is 4 bytes inside a block of size 6 alloc'd
==40399==    at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==40399==    by 0x10923E: split_array (split_array.c:19)
==40399==    by 0x1093B5: main (split_array.c:59)
