Error processing PDB 4q4h

While testing JAL-1829, I discovered that the StructureSelectionManager class throws an ArrayIndexOutOfBoundsException while attempting to load some PDB structures concurrently, this happens on both JMol and Chimera

To reproduce:
- fetch the following sequences from PDB: 3nh6; 4rvc; 4q4h.
- select all of the sequence and attempt to view all structures for them together.

Stack trace for JMol:
Squashed Jmol callback handler error:
java.lang.ArrayIndexOutOfBoundsException: -2
at jalview.structure.StructureSelectionManager.setMapping(StructureSelectionManager.java:521)
at jalview.structure.StructureSelectionManager.setMapping(StructureSelectionManager.java:323)
at jalview.ext.jmol.JalviewJmolBinding.notifyFileLoaded(JalviewJmolBinding.java:1185)
at jalview.ext.jmol.JalviewJmolBinding.notifyCallback(JalviewJmolBinding.java:995)
at org.jmol.viewer.StatusManager.setFileLoadStatus(Unknown Source)
at org.jmol.viewer.Viewer.setFileLoadStatus(Unknown Source)
at org.jmol.viewer.Viewer.createModelSetAndReturnError(Unknown Source)
at org.jmol.viewer.Viewer.loadModelFromFile(Unknown Source)
at org.jmol.script.ScriptEval.cmdLoad(Unknown Source)
at org.jmol.script.ScriptEval.processCommand(Unknown Source)
at org.jmol.script.ScriptEval.commandLoop(Unknown Source)
at org.jmol.script.ScriptEval.dispatchCommands(Unknown Source)
at org.jmol.script.ScriptEval.executeCommands(Unknown Source)
at org.jmol.script.ScriptEval.evaluateCompiledScript(Unknown Source)
at org.jmol.script.ScriptManager.evalStringWaitStatusQueued(Unknown Source)
at org.jmol.viewer.Viewer.evalStringWaitStatusQueued(Unknown Source)
at org.jmol.script.ScriptQueueThread.runNextScript(Unknown Source)
at org.jmol.script.ScriptQueueThread.run1(Unknown Source)
at org.jmol.thread.JmolThread.run(Unknown Source)


Stack trace for Chimera:
ERROR: jalview.bin.Jalview - Couldn't open /var/folders/yk/7sbd4m7x5gbf5_lx4kz8rd99t8z5dm/T/jalview2451205814476899932.xml in Chimera viewer!
java.lang.ArrayIndexOutOfBoundsException: -2
at jalview.structure.StructureSelectionManager.setMapping(StructureSelectionManager.java:521)
at jalview.structure.StructureSelectionManager.setMapping(StructureSelectionManager.java:323)
at jalview.gui.ChimeraViewFrame.run(ChimeraViewFrame.java:861)
at java.lang.Thread.run(Thread.java:745)
ERROR - Couldn't open /var/folders/yk/7sbd4m7x5gbf5_lx4kz8rd99t8z5dm/T/jalview2451205814476899932.xml in Chimera viewer!
java.lang.ArrayIndexOutOfBoundsException: -2
at jalview.structure.StructureSelectionManager.setMapping(StructureSelectionManager.java:521)
at jalview.structure.StructureSelectionManager.setMapping(StructureSelectionManager.java:323)
at jalview.gui.ChimeraViewFrame.run(ChimeraViewFrame.java:861)
at java.lang.Thread.run(Thread.java:745)