Fix base64 decode zero-length dyn. buffer handling
Fix base64 decode zero-length dyn. buffer handling

Base64 fast path decoder failed to handle a zero-length input when input
pointer was NULL.  This happens for dynamic buffers (only).  The decoder
fails because pointer arithmetic is performed on the NULL.

Fix the issue by changing duk__prep_codec_arg() to always guarantee a
non-NULL return which simplifies call site assumptions.

svaarala/duktape,svaarala/duktape,svaarala/duktape,svaarala/duktape,svaarala/duktape,svaarala/duktape,svaarala/duktape,svaarala/duktape,svaarala/duktape