Don't crash in wxTextFile::GetLastLine() if the file is empty

Just return a reference to an empty wxString, as GetFirstLine() already does
(although this is actually questionable -- what if this string is modified?).

See #17283.

docs/changes.txt

@@ -73,6 +73,7 @@ All:
 - Make it easier to convert to/from UTF-8-encoded std::string (ARATA Mizuki).
 - Add support for loading dynamic lexer in wxStyledTextCtrl (New Pagodi).
 - Handle strings with embedded NULs in wxDataStream (Nitch).
+- Don't crash in wxTextFile::GetLastLine() if the file is empty (crohr).
 
 All (GUI):
 

include/wx/textbuf.h

@@ -125,7 +125,7 @@ public:
     wxString& GetPrevLine()  /* const */
         { wxASSERT(m_nCurLine > 0); return m_aLines[--m_nCurLine]; }
     wxString& GetLastLine() /* const */
-        { m_nCurLine = m_aLines.size() - 1; return m_aLines.Last(); }
+        { return m_aLines.empty() ? ms_eof : m_aLines[m_nCurLine = m_aLines.size() - 1]; }
 
     // get the type of the line (see also GetEOL)
     wxTextFileType GetLineType(size_t n) const { return m_aTypes[n]; }

tests/textfile/textfiletest.cpp

@@ -117,6 +117,9 @@ void TextFileTestCase::ReadEmpty()
     CPPUNIT_ASSERT( f.Open(wxString::FromAscii(GetTestFileName())) );
 
     CPPUNIT_ASSERT_EQUAL( (size_t)0, f.GetLineCount() );
+    CPPUNIT_ASSERT( f.Eof() );
+    CPPUNIT_ASSERT_EQUAL( "", f.GetFirstLine() );
+    CPPUNIT_ASSERT_EQUAL( "", f.GetLastLine() );
 }
 
 void TextFileTestCase::ReadDOS()