-> Fixed crasher in extrude code

New extrude code was accessing an uninitialized
variable. Why this sort of thing doesn't cause
crashes on windows, I dont have a clue!

Note for Joe: I see a lot of 'logic' going on in
the client code for extrude that should possibly
put inside the BMOP system itself (aside from the
part about modifiers). This should be cleaned up
in future maybe...

source/blender/editors/mesh/editmesh_lib.c

@@ -1178,6 +1178,7 @@ short BM_extrude_edgeflag(Object *obedit, BMesh *bm, int eflag, float *nor)
 	/* If a mirror modifier with clipping is on, we need to adjust some 
 	 * of the cases above to handle edges on the line of symmetry.
 	 */
+	md = obedit->modifiers.first;
 	for (; md; md=md->next) {
 		if (md->type==eModifierType_Mirror) {
 			MirrorModifierData *mmd = (MirrorModifierData*) md;