[regexp] Fix incorrect DCHECK in FixSingleCharacterDisjunctions

test/mjsunit/regexp.js

 // Copyright 2012 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
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@@ skipping 785 matching lines @@
 assertTrue(/^[\7]*$/.test("\u{7}"));  // ... ClassEscape
                                       // ~~> LegacyOctalEscapeSequence.
 assertTrue(/^[\11]*$/.test("\u{9}"));
 assertTrue(/^[\111]*$/.test("\u{49}"));
 assertTrue(/^[\222]*$/.test("\u{92}"));
 assertTrue(/^[\333]*$/.test("\u{DB}"));
 assertTrue(/^[\444]*$/.test("\u{24}4"));
 assertTrue(/^[\d-X]*$/.test("234-X-432"));  // CharacterRangeOrUnion.
 assertTrue(/^[\d-X-Z]*$/.test("234-XZ-432"));
 assertFalse(/^[\d-X-Z]*$/.test("234-XYZ-432"));
+
+// Lone leading surrogates. Just here to exercise specific parsing code-paths.
+
+assertFalse(/\uDB88|\uDBEC|aa/.test(""));
+assertFalse(/\uDB88|\uDBEC|aa/u.test(""));