Missing String bounds checks

[nirvdrum]

Strings in JRuby are currently limited to signed int length. String#* expressly checks for this:

> ./bin/jruby -v -e 'p "blah" * 1_000_000_000'
jruby 9.1.0.0-SNAPSHOT (2.3.0) 2016-04-11 f79afda Java HotSpot(TM) 64-Bit Server VM 25.77-b03 on 1.8.0_77-b03 +jit [linux-x86_64]
ArgumentError: argument too big
      * at org/jruby/RubyString.java:1161
  <top> at -e:1

However, this bound check isn't carried through to other construction operations. E.g., String#+ ends up wrapping around and a negative length array is created:

> bin/jruby -J-Xmx8g -v -e 'x = "f" * (2**31 - 3); p (x + x).size'
jruby 9.1.0.0-SNAPSHOT (2.3.0) 2016-04-11 f79afda Java HotSpot(TM) 64-Bit Server VM 25.77-b03 on 1.8.0_77-b03 +jit [linux-x86_64]
Unhandled Java exception: java.lang.NegativeArraySizeException
java.lang.NegativeArraySizeException: null
               <init> at org/jruby/util/ByteList.java:91
         addByteLists at org/jruby/util/StringSupport.java:1630
            op_plus19 at org/jruby/RubyString.java:1149
         cacheAndCall at org/jruby/runtime/callsite/CachingCallSite.java:313
                 call at org/jruby/runtime/callsite/CachingCallSite.java:163
                <top> at -e:1
  invokeWithArguments at java/lang/invoke/MethodHandle.java:627
                 load at org/jruby/ir/Compiler.java:111
            runScript at org/jruby/Ruby.java:825
            runScript at org/jruby/Ruby.java:817
          runNormally at org/jruby/Ruby.java:755
          runFromMain at org/jruby/Ruby.java:576
        doRunFromMain at org/jruby/Main.java:425
          internalRun at org/jruby/Main.java:310
                  run at org/jruby/Main.java:239
                 main at org/jruby/Main.java:201