This commit adds the requirement that matrices being multiplied together

aren't empty.  Previously this was technically allowed and worked but was
weird.  The optimization I checked a few hours ago also breaks when empty
matrices are multiplied together so I'm just adding this new requirement.

I also had to fix a part of the LU decomposition because it was doing an
empty matrix multiplication at one point.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403104

dlib/matrix/matrix.h

@@ -331,7 +331,7 @@ namespace dlib
             // You are trying to multiply two incompatible matrices together.  The number of columns 
             // in the matrix on the left must match the number of rows in the matrix on the right.
             COMPILE_TIME_ASSERT(LHS::NC == RHS::NR || LHS::NC*RHS::NR == 0);
-            DLIB_ASSERT(lhs.nc() == rhs.nr(), 
+            DLIB_ASSERT(lhs.nc() == rhs.nr() && lhs.size() > 0 && rhs.size() > 0, 
                 "\tconst matrix_exp operator*(const matrix_exp& lhs, const matrix_exp& rhs)"
                 << "\n\tYou are trying to multiply two incompatible matrices together"
                 << "\n\tlhs.nr(): " << lhs.nr()

dlib/matrix/matrix_abstract.h

@@ -196,6 +196,8 @@ namespace dlib
     /*!
         requires
             - m1.nc() == m2.nr()
+            - m1.size() > 0 && m2.size() > 0
+              (you can't multiply any sort of empty matrices together)
             - m1 and m2 both contain elements of the same type
         ensures
             - returns the result of doing the matrix multiplication m1*m2.  The resulting

dlib/matrix/matrix_lu.h

@@ -128,7 +128,11 @@ namespace dlib
             {
                 // Most of the time is spent in the following dot product.
                 const long kmax = std::min(i,j);
-                const type s = rowm(LU,i, kmax)*colm(LUcolj,0,kmax);
+                type s;
+                if (kmax > 0)
+                    s = rowm(LU,i, kmax)*colm(LUcolj,0,kmax);
+                else 
+                    s = 0;
 
                 LU(i,j) = LUcolj(i) -= s;
             }