utils.c: NULL terminate ast_base64decode_string.

With the addition of STIR/SHAKEN, the function ast_base64decode_string
was added for convenience since there is a lot of converting done during
the STIR/SHAKEN process. This function returned the decoded string for
you, but did not NULL terminate it, causing some issues (specifically
with MALLOC_DEBUG). Now, the returned string is NULL terminated, and the
documentation has been updated to reflect this.

Change-Id: Icdd7d05b323b0c47ff6ed43492937a03641bdcf5