FreeBSD build warning in aes_new_cipher()

Our jenkins freebsd builders are trying again! And they are failing with:

18:20:41 ../tor/src/common/aes.c:107:14: error: variable 'c' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
18:20:41     default: tor_assert(0); // LCOV_EXCL_LINE
18:20:41              ^~~~~~~~~~~~~

Initializing c to NULL should solve this problem, which is IMO the compiler's fault.

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