One less use of NSPR is a good thing. The failure cases that
PL_Base64Encode would have caught for us are:
1. "Truncation".
2. Integer overflow when computing destination string length.
3. Malloc failures.
The first one only gets checked if we pass in zero for the source
length, which we never do. The latter two only get checked if we pass
in a null pointer for the destination, which we never do. So removing
the error handling PL_Base64Encode implies here is a good thing.
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