Fixed a bug in an assert statement and also changed the code to avoid

potential compiler warnings.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402868

dlib/matrix/matrix.h

@@ -98,7 +98,7 @@ namespace dlib
             long c
         ) const 
         { 
-            DLIB_ASSERT(r < nr() && c < nc() && r >= 0 && c >= c, 
+            DLIB_ASSERT(r < nr() && c < nc() && r >= 0 && c >= 0, 
                 "\tconst type matrix_exp::operator(r,c)"
                 << "\n\tYou must give a valid row and column"
                 << "\n\tr:    " << r 

dlib/matrix/matrix_expressions.h

@@ -1498,7 +1498,7 @@ namespace dlib
 
         long operator() (
             long r, 
-            long c
+            long 
         ) const { return start + r*inc;  }
 
         long operator() (