Fix NPE in TemporaryBuffer when writing to closed buffer

[pstreef]

Problem

Large repository operations that exceed the buffer's in-core limit can trigger a NullPointerException in TemporaryBuffer.write():

java.lang.NullPointerException: Cannot invoke "java.util.ArrayList.size()" because "this.blocks" is null
    at org.openrewrite.jgit.util.TemporaryBuffer.last(TemporaryBuffer.java:352)
    at org.openrewrite.jgit.util.TemporaryBuffer.write(TemporaryBuffer.java:111)
    at org.openrewrite.jgit.transport.PacketLineOut.end(PacketLineOut.java:203)
    at org.openrewrite.jgit.transport.BasePackConnection.close(BasePackConnection.java:652)

This happens when:

1. Buffer exceeds in-core limit → switchToOverflow() sets blocks = null
2. Transport operation fails
3. close() is called, setting overflow = null
4. Error cleanup calls pckOut.end() which tries to write to the closed buffer
5. NPE when accessing blocks.size()

The NPE masks the original transport error because it's thrown during cleanup.

Solution

Add defensive checks in write() methods to throw IOException instead of NPE when the buffer is in a closed state (both blocks and overflow are null).

Why this works

The key insight is in BasePackConnection.close():

try {
    if (outNeedsEnd) {
        pckOut.end();  // throws IOException now, not NPE
    }
} catch (IOException err) {
    // Ignore any close errors
}

• NPE (RuntimeException): Not caught → bubbles up and masks the original error
• IOException: Caught and ignored → close() completes → original error propagates

This allows the actual transport failure to be reported instead of the cryptic NPE.