Fix a DRC crash on some installs.

It looks like D_GLIBCXX_ASSERTIONS=ON activated on some Linux versions creates too zealous tests at run time.
This patch just calculate the same pointer value using a slightly different formula.

Fix also a hard-to-understand comment

Fixes: lp:1770414
https://bugs.launchpad.net/kicad/+bug/1770414

pcbnew/drc.cpp

@@ -633,6 +633,9 @@ void DRC::testPad2Pad()
 
     m_pcb->GetSortedPadListByXthenYCoord( sortedPads );
 
+    if( sortedPads.size() == 0 )
+        return;
+
     // find the max size of the pads (used to stop the test)
     int max_size = 0;
 
@@ -647,9 +650,10 @@ void DRC::testPad2Pad()
             max_size = radius;
     }
 
-    // Test the pads
-    D_PAD** listEnd = &sortedPads[ sortedPads.size() ];
+    // Upper limit of pad list (limit not included)
+    D_PAD** listEnd = &sortedPads[0] + sortedPads.size();
 
+    // Test the pads
     for( unsigned i = 0; i< sortedPads.size(); ++i )
     {
         D_PAD* pad = sortedPads[i];

pcbnew/drc.h

@@ -311,10 +311,12 @@ private:
      *
      * The pad list must be sorted by x coordinate.
      *
-     * @param aRefPad The pad to test
-     * @param aStart The start of the pad list to test against
-     * @param aEnd Marks the end of the list and is not included
-     * @param x_limit is used to stop the test (when the any pad's X coord exceeds this)
+     * @param aRefPad is the pad to test
+     * @param aStart is the first pad of the list to test against aRefPad
+     * @param aEnd is the end of the list and is not included
+     * @param x_limit is used to stop the test
+     * (i.e. when the current pad pos X in list exceeds this limit, because the list
+     * is sorted by X coordinate)
      */
     bool doPadToPadsDrc( D_PAD* aRefPad, D_PAD** aStart, D_PAD** aEnd, int x_limit );
 
@@ -322,7 +324,7 @@ private:
      * Test the current segment.
      *
      * @param aRefSeg The segment to test
-     * @param aStart The head of a list of tracks to test against (usually BOARD::m_Track)
+     * @param aStart the first item of track list to test against (usually BOARD::m_Track)
      * @param doPads true if should do pads test
      * @return bool - true if no problems, else false and m_currentMarker is
      *          filled in with the problem information.