StringIndexOutOfBoundsException when printing warnings in transformed files

AMP is seeing an exception being thrown when printing a code warning: https://github.com/ampproject/amp-closure-compiler/issues/22

```
java.lang.StringIndexOutOfBoundsException: String index out of range: 75
at java.lang.StringLatin1.charAt(StringLatin1.java:47)
at java.lang.String.charAt(String.java:693)
at com.google.javascript.jscomp.LightweightMessageFormatter.padLine(LightweightMessageFormatter.java:197)
at com.google.javascript.jscomp.LightweightMessageFormatter.padMultipleLines(LightweightMessageFormatter.java:251)
at com.google.javascript.jscomp.LightweightMessageFormatter.getExcerptWithPosition(LightweightMessageFormatter.java:161)
at com.google.javascript.jscomp.LightweightMessageFormatter.format(LightweightMessageFormatter.java:133)
at com.google.javascript.jscomp.LightweightMessageFormatter.formatWarning(LightweightMessageFormatter.java:86)
at com.google.javascript.jscomp.JSError.format(JSError.java:171)
at com.google.javascript.jscomp.PrintStreamErrorReportGenerator.println(PrintStreamErrorReportGenerator.java:56)
at com.google.javascript.jscomp.PrintStreamErrorReportGenerator.generateReport(PrintStreamErrorReportGenerator.java:50)
at com.google.javascript.jscomp.SortingErrorManager.generateReport(SortingErrorManager.java:116)
at com.google.javascript.jscomp.ThreadSafeDelegatingErrorManager.generateReport(ThreadSafeDelegatingErrorManager.java:40)
at com.google.javascript.jscomp.Compiler.generateReport(Compiler.java:724)
at com.google.javascript.jscomp.AbstractCommandLineRunner.performFullCompilation(AbstractCommandLineRunner.java:1336)
at com.google.javascript.jscomp.AbstractCommandLineRunner.doRun(AbstractCommandLineRunner.java:1254)
at com.google.javascript.jscomp.AbstractCommandLineRunner.run(AbstractCommandLineRunner.java:540)
at org.ampproject.AmpCommandLineRunner.main(Unknown Source)
```

After a bit of tracking, we've managed to isolate it down to the file's input sourcemap causing a confusion in the source code excerpt printer. Essentially, it's trying to print the transformed file's source code using locations from the original source. This is because the transformed code and the original source file share the same file name (the code transform is happening in memory with Gulp before passing it to Closure).

I've made an easy reproduction at https://github.com/jridgewell/closure-warn-sourcemaps-oob.