handle problem when box is to small to show any text

Signed-off-by: Florian Thienel <florian@thienel.org>
diff --git a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/TextContent.java b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/TextContent.java
index b450579..22f2bdc 100644
--- a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/TextContent.java
+++ b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/TextContent.java
@@ -125,6 +125,9 @@
 	}
 
 	public String getText() {
+		if (startPosition.getOffset() > endPosition.getOffset()) {
+			return "";
+		}
 		return content.getText(new ContentRange(startPosition.getOffset(), endPosition.getOffset()));
 	}