your buffer is not large enough, and as a result you are writing out-of-bounds and corrupting the tests suite.
note that printf("%d", INT_MIN) == 11

Look at begining of your function. Empty array is {}

{{}} - array containing 1 empty array.

;)

I noticed that too. Sometimes my solution pass the random tests and sometimes it results in incorrect answer for extremely large tests where exponent is small enough.
UPD: Tested my solution against reference solution with large base/modulo and exponent between 1/100. It's guaranteed that in the case exponent >= 1 <= 7 my solution fails.

invalid input I got was:
-1
0
-101

You're welcome

You have exactly one way to give out 0 change: give out 0 coins. This is consistent with the binomial coefficient in mathmatics, which gives n over 0 = 1 (i.e. you have 1 possiblity to choose 0 elements from n).