C++: prevent possible nullptr deref.

Change-Id: Ibc138bcfdef4b6a5dd154e819d568398ecace982
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/semantichighlighter.cpp

@@ -148,6 +148,8 @@ unsigned SemanticHighlighter::documentRevision() const
 
 void SemanticHighlighter::updateFormatMapFromFontSettings()
 {
+    QTC_ASSERT(m_baseTextDocument, return);
+
     const TextEditor::FontSettings &fs = m_baseTextDocument->fontSettings();
 
     m_formatMap[TypeUse] = fs.toTextCharFormat(TextEditor::C_TYPE);