CID 26257: Null pointer dereferences (NULL_RETURNS)

this test should never fire, but it shuts up coverity

src/spells/combatspells.c

@@ -622,11 +622,13 @@ int sp_mindblast(struct castorder * co)
                 skill_t sk = random_skill(du, false);
                 if (sk != NOSKILL) {
                     skill *sv = unit_skill(du, sk);
+                    if (sv) {
                         int n = 1 + rng_int() % 3;
 
                         reduce_skill(du, sv, n);
                         k += du->number;
                     }
+                }
                 else {
                     /* unit has no skill. kill it. */
                     kill_troop(dt);