error loading object file when "usemtl" references an undefined material

SConstruct

@@ -1,6 +1,6 @@
 
 # vim:filetype=python
 
-env = Environment(LIBS = ['GL'], CXXFLAGS = ['-Wall'])
+env = Environment(LIBS = ['GL'], CXXFLAGS = ['-Wall', '-DGL_GLEXT_PROTOTYPES'])
 
 env.Program('driver', Glob('*.cc'))

WFObj.cc

@@ -1,6 +1,4 @@
 
-#define GL_GLEXT_PROTOTYPES
-
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
@@ -464,6 +462,12 @@ bool WFObj::buildVBO()
             it != m_faces.end();
             it++)
     {
+        if (m_materials.find(it->first) == m_materials.end())
+        {
+            cerr << "Material '" << it->first << "' referenced in '"
+                << m_path << "' is not defined." << endl;
+            return false;
+        }
         for (vector<Face>::iterator fit = it->second.begin();
                 fit != it->second.end();
                 fit++)

WFObj.h

@@ -19,7 +19,7 @@ public:
     class Buffer
     {
         public:
-            Buffer() : m_alloc(false) {}
+            Buffer() : m_alloc(false) {data = NULL; length = 0;}
             ~Buffer() { if (m_alloc) delete data; }
             uint8_t *data;
             size_t length;