Fix T70150: Explode modifier crashes with empty vertex group.

There’s be a lot to say about that explode modifier code, for now just
follow other places in code... But the handling of 'invalid'/'unkown'
particle case is... quiet confusing to say the least.

source/blender/modifiers/intern/MOD_explode.c

@@ -1069,7 +1069,7 @@ static Mesh *explodeMesh(ExplodeModifierData *emd,
 
     /* override uv channel for particle age */
     if (mtface) {
-      float age = (cfra - pa->time) / pa->lifetime;
+      float age = (facepa[i] != totpart) ? (cfra - pa->time) / pa->lifetime : 0.0f;
       /* Clamp to this range to avoid flipping to the other side of the coordinates. */
       CLAMP(age, 0.001f, 0.999f);