Base64Encode fails if the 'out_len' output parameter is passed with certain values

The Base64Encode function in coding-util.h with signature 

bool Base64Encode(const char* in, int64_t in_len, int64_t out_max, char* out, int64_t* out_len);

fails if '*out_len', when passed to the function, contains a negative value or a value that does not fit in a 32 bit integer.

Internally we use the 
 

int sasl_encode64(const char *in, unsigned inlen, char *out, unsigned outmax, unsigned *outlen);

 
function and explicitly cast 'out_len' to 'unsigned*'.

The success of this function shouldn't depend on the value of '*out_len' because it is an output parameter, so we should set '*out_len' to zero before passing it to sasl_encode64().