RFR: 8371309: Diagnostic.getEndPosition can throw an NPE with typical broken code

[Jan Lahoda]

When starting the compiler via API, a `DiagnosticListener` may be provided, to which all reported errors and warnings are reported.

The reported diagnostics are represented by `javax.tools.Diagnostic`, which has (among others) method `getEndPosition`, which requires the end-pos-table to be set in the `DiagnosticSource` object for the given file (see `AbstractLog.sourceMap`). But the end-pos-table is only set after the parse is done:
https://github.com/openjdk/jdk/blob/167c952bb0fefb5acc9782f4f4474d92097c93f8/src/jdk.compiler/share/classes/com/sun/tools/javac/main/JavaCompiler.java#L702

There are three problems with this:
- when a `Diagnostic` is reported by the parser, and the clients code calls `Diagnostic.getEndPosition`, the end-pos-table is not set yet, and the method crashes with an NPE
- the end-pos-table is never set for implicitly compiled (parsed) files:
https://github.com/openjdk/jdk/blob/167c952bb0fefb5acc9782f4f4474d92097c93f8/src/jdk.compiler/share/classes/com/sun/tools/javac/main/JavaCompiler.java#L840
- when a `Diagnostic` is reported for a tree by the parser, the tree on which the `Diagnostic` appear does not typically have the end position set yet - it is usually set only after the error/warning is reported

All those problems used to be hidden by this check:
https://github.com/openjdk/jdk/blob/6c48f4ed707bf0b15f9b6098de30db8aae6fa40f/src/jdk.compiler/share/classes/com/sun/tools/javac/tree/TreeInfo.java#L649
but that has been removed:
https://github.com/openjdk/jdk/commit/c793de989facdb532021e1d5ddd01eb0e089b8e6

I have tried to fix this properly, by deferring the errors from the parser to a later point, properly setting the end-pos-table, and handling implicitly parsed files more correctly:
https://github.com/openjdk/jdk/compare/master...lahodaj:jdk:JDK-8371309b?expand=1
but this change is a bit involved, and potentially a bit dangerous.

So, for now, I propose to reinstate the null check in `TreeInfo.getEndPos`, to restore the JDK 25 behavior, and attempt to do the better solution later, hopefully earlier in the release cycle.

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Commit messages:
 - 8371309: Diagnostic.getEndPosition can throw an NPE with typical broken code

Changes: https://git.openjdk.org/jdk/pull/28316/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28316&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371309
  Stats: 194 lines in 2 files changed: 194 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28316.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28316/head:pull/28316

PR: https://git.openjdk.org/jdk/pull/28316