func_aes: fix misuse of strlen on binary data

The encryption code for AES_ENCRYPT evaluates the length of the data to
be encoded in base64 using strlen. The data is binary, thus the length
of it can be underestimated at the first NULL character.
Reuse the write pointer offset to evaluate it, instead.

ASTERISK-25857 #close

Change-Id: If686b5d570473eb926693c73461177b35b13b186