[TD]protect against bad pref value

- this is a temporary measure to prevent problems caused by a
  bad value for LineStandard parameter.  A previous devel version
  stored on invalid value.  This patch can be removed before
  moving to production.
- this condition can be corrected by editing LineStandard to 0, 1 or
  2.  a plethora of warning messages is issued until the parameter is
  corrected.

src/Mod/TechDraw/App/Preferences.cpp

@@ -23,7 +23,7 @@
 #include "PreCompiled.h"
 #ifndef _PreComp_
 # include <string>
-
+# include <QApplication>
 # include <QString>
 #endif
 
@@ -422,6 +422,18 @@ bool Preferences::SectionUsePreviousCut()
 //! an index into the list of available line standards/version found in LineGroupDirectory
 int Preferences::lineStandard()
 {
+    // there is a condition where the LineStandard parameter exists, but is -1 (the
+    // qt value for no current index in a combobox).  This is likely caused by an old
+    // development version writing an unvalidated value.  In this case, the
+    // existing but invalid value will be returned.  This is a temporary fix and
+    // can be removed for production.
+    // this message will appear many times if the parameter is invalid.
+    int parameterValue = getPreferenceGroup("Standards")->GetInt("LineStandard", 1);
+    if (parameterValue < 0) {
+        Base::Console().Warning(qPrintable(QApplication::translate(
+        "Preferences", "The LineStandard parameter is invalid. Using zero instead.", nullptr)));
+        return 0;
+    }
     return getPreferenceGroup("Standards")->GetInt("LineStandard", 1);
 }