[analyzer] Fix StdLibraryFunctionsChecker crash on surprising sink node (#66109)

Recent changes in StdLibraryFunctionsChecker introduced a situation
where the checker sequentially performed two state transitions to add
two separate note tags.

In the unlikely case when the updated state (the variable `NewState`)
was posteriorly overconstrained, the engine marked the node after the
first state transition as a sink to stop the "natural" graph exploration
after that point.

However, in this particular case the checker tried to directly add a
second node, and this triggered an assertion in the `addPredecessor()`
method of `ExplodedNode`.

This commit introduces an explicit `isSink()` check to avoid this crash.
To avoid similar bugs in the future, perhaps it would be possible to
tweak `addTransition()` and ensure that it returns `nullptr` when it
would return a sink node (to unify the two possible error conditions).

This crash was observed in an analysis of the curl project (in a very
long and complex function), and there I validated that this is the root
cause, but I don't have a self-contained testcase that can trigger the
creation of a PosteriorlyOverconstrained node in this situation.

Author: NagyDonat

clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp

@@ -1387,8 +1387,8 @@ void StdLibraryFunctionsChecker::checkPostCall(const CallEvent &Call,
     if (!NewState)
       continue;
 
-    // It is possible that NewState == State is true.
-    // It can occur if another checker has applied the state before us.
+    // Here it's possible that NewState == State, e.g. when other checkers
+    // already applied the same constraints (or stricter ones).
     // Still add these note tags, the other checker should add only its
     // specialized note tags. These general note tags are handled always by
     // StdLibraryFunctionsChecker.
@@ -1427,7 +1427,12 @@ void StdLibraryFunctionsChecker::checkPostCall(const CallEvent &Call,
             });
         Pred = C.addTransition(NewState, Pred, Tag);
       }
-      if (!Pred)
+
+      // Pred may be:
+      //  - a nullpointer, if we reach an already existing node (theoretically);
+      //  - a sink, when NewState is posteriorly overconstrained.
+      // In these situations we cannot add the errno note tag.
+      if (!Pred || Pred->isSink())
         continue;
     }