base64: don't drop already-decoded output when decoding hits invalid input

[Chaganti-Reddy]

base64 -d was throwing away everything it had already decoded whenever it hit invalid input later in the stream, instead of writing out the good prefix like GNU does. root cause was two spots that only flushed whole internal batches instead of whatever complete groups were actually buffered, plus base64's decoder rejecting a trailing group with non-canonical padding bits that GNU still accepts.

Fixes #12923

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will not alter performance

✅ 331 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing Chaganti-Reddy:base64-partial-decode-on-error (0fec3f0) with main (ff3f37d)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩