util/var: add NULL check in VarNameStoreRegister

And check return value in entropy keyword setup

src/detect-entropy.c

@@ -56,6 +56,9 @@ static int DetectEntropySetup(DetectEngineCtx *de_ctx, Signature *s, const char
     } else {
         ded->fv_idx = VarNameStoreRegister("content", VAR_TYPE_FLOW_FLOAT);
     }
+    if (ded->fv_idx == 0) {
+        goto error;
+    }
 
     if (SCSigMatchAppendSMToList(de_ctx, s, DETECT_ENTROPY, (SigMatchCtx *)ded, sm_list) != NULL) {
         SCReturnInt(0);

src/util-var-name.c

@@ -154,6 +154,9 @@ void VarNameStoreDestroy(void)
  */
 uint32_t VarNameStoreRegister(const char *name, const enum VarTypes type)
 {
+    if (name == NULL) {
+        return 0;
+    }
     SCMutexLock(&base_lock);
     uint32_t id = 0;