C doesn’t check array boundaries. A segmentation fault will only occur if you try to dereference a pointer to memory that your program doesn’t have permission to access. Simply going past the end of an array is unlikely to cause that behaviour. Undefined behaviour is just that – undefined. It may appear to work just fine, but you shouldn’t be relying on its safety.
Your program causes undefined behaviour by accessing memory past the end of the array. In this case, it looks like one of your str[i] = c writes overwrites the value in i.
