Silence a warning and add an assertion

Another false positive uninitialized variable. Actually it's only false
if ToOutputVertexIndex does't return NULL which should only happen if
index is out of bounds. Add assertion to make sure of that.

code/FBXConverter.cpp

@@ -1256,8 +1256,11 @@ private:
 				// taking notes so we don't need to do it twice.
 				BOOST_FOREACH(WeightIndexArray::value_type index, indices) {
 
-					unsigned int count;
+					unsigned int count = 0;
 					const unsigned int* const out_idx = geo.ToOutputVertexIndex(index, count);
+					// ToOutputVertexIndex only returns NULL if index is out of bounds
+					// which should never happen
+					ai_assert(out_idx != NULL);
 
 					index_out_indices.push_back(no_index_sentinel);
 					count_out_indices.push_back(0);