Cleanup: MOD_explode: Sanitize code some more.

This happens to be a non-behavioral change, but previous code here was
*very* confusing, and only ended up generating expected results by mere
chance (since `facepa[i]` == `totpart` in case the face has no (valid)
particle, i.e. `pa == NULL`)...

source/blender/modifiers/intern/MOD_explode.c

@@ -1058,7 +1058,8 @@ static Mesh *explodeMesh(ExplodeModifierData *emd,
 
     orig_v4 = source.v4;
 
-    if (pa != NULL && cfra < pa->time) {
+    /* Same as above in the first loop over mesh's faces. */
+    if (pa == NULL || cfra < pa->time) {
       mindex = totvert + totpart;
     }
     else {