Allocate more space for the wxRegEx error message buffer

We need to account for the trailing NUL explicitly here, so add 1 to the
length returned by the first call to wx_regerror() to avoid chopping off
the last character of the error message.

src/common/regex.cpp

@@ -444,7 +444,7 @@ wxString wxRegExImpl::GetErrorMsg(int errorcode) const
     int len = wx_regerror(errorcode, &m_RegEx, NULL, 0);
     if ( len > 0 )
     {
-        wxCharTypeBuffer<wxRegErrorChar> errbuf(len);
+        wxCharTypeBuffer<wxRegErrorChar> errbuf(len + 1);
 
         (void)wx_regerror(errorcode, &m_RegEx, errbuf.data(), errbuf.length());