RFR: 8037965: NullPointerException in TextLayout.getBaselineFromGraphic() for JTextComponents

[Prasanta Sadhukhan]

When invalid unicode codepoints 0xFFFF and 0x10000 are added to an empty javax.swing.JTextComponent (JTextArea or JTextField), the view cannot be updated due to a NullPointerException in TextLayout.getBaselineFromGraphic() 
owing to the fact GraphicAttribute is null for these invalid codepoints. 

TextLayout.getBaselineFromGraphic() is called from [TextLayout.standardInit](https://github.com/openjdk/jdk/blame/master/src/java.desktop/share/classes/java/awt/font/TextLayout.java#L647) and [TextMeasurer.initAll](https://github.com/openjdk/jdk/blame/master/src/java.desktop/share/classes/java/awt/font/TextMeasurer.java#L249) where it relies on CHAR_REPLACEMENT TextAttribute 
and its spec says "The default value is null, indicating that the standard glyphs provided by the font should be used. " so if invalid codepoints is used, the default value is not changed so we need to be ready for null value too.

Since TextAttribute_CHAR_REPLACEMENT says "It is required for correct positioning of 'inline' components within a line when bidirectional reordering is performed. Each character (Unicode code point) will be rendered using the provided GraphicAttribute." so 
Fixed by checking if GraphicAttribute is null, then keep the baseline to default value (which is 0) by returning default baseline (which is ROMAN_BASELINE)

Existing jtreg,jck tests are green.

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Commit messages:
 - 8037965: NullPointerException in TextLayout.getBaselineFromGraphic() for JTextComponents
 - 8037965: NullPointerException in TextLayout.getBaselineFromGraphic() for JTextComponents

Changes: https://git.openjdk.java.net/jdk/pull/7490/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7490&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8037965
  Stats: 41 lines in 2 files changed: 41 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7490.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7490/head:pull/7490

PR: https://git.openjdk.java.net/jdk/pull/7490