Fix write behind allocated memory in thread name setting (#34424)

The code in CorUnix::InternalSetThreadDescription is writing behind
the end of the allocated memory in case the name is shorter than 16
characters. That is causing memory heap corruption.

src/coreclr/src/pal/src/thread/thread.cpp

@@ -1666,7 +1666,10 @@ CorUnix::InternalSetThreadDescription(
 
     // Null terminate early.
     // pthread_setname_np only accepts up to 16 chars.
-    nameBuf[15] = '\0';
+    if (nameSize > 15)
+    {
+        nameBuf[15] = '\0';
+    }
 
     error = pthread_setname_np(pTargetThread->GetPThreadSelf(), nameBuf);