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

funcs/func_aes.c

@@ -146,7 +146,7 @@ static int aes_helper(struct ast_channel *chan, const char *cmd, char *data,
 	}
 
 	if (encrypt) {                            /* if encrypting encode result to base64 */
-		ast_base64encode(buf, (unsigned char *) tmp, strlen(tmp), len);
+		ast_base64encode(buf, (unsigned char *) tmp, tmpP - tmp, len);
 	} else {
 		memcpy(buf, tmp, len);
 	}