logging: prevent double-free multi-threaded logging

When multithreaded output was enabled, all threads freed
shallow-copy of sensor and prefix names.

This commit adds a guard so that only the parent frees the
allocated buffers.

Ticket: 8861

src/util-logopenfile.c

@@ -983,17 +983,20 @@ int LogFileFreeCtx(LogFileCtx *lf_ctx)
         SCMutexDestroy(&lf_ctx->fp_mutex);
     }
 
-    if (lf_ctx->prefix != NULL) {
-        SCFree(lf_ctx->prefix);
-        lf_ctx->prefix_len = 0;
+    if (lf_ctx->parent == NULL) {
+        if (lf_ctx->prefix != NULL) {
+            SCFree(lf_ctx->prefix);
+            lf_ctx->prefix_len = 0;
+        }
+
+        if (lf_ctx->sensor_name != NULL) {
+            SCFree(lf_ctx->sensor_name);
+        }
     }
 
     if(lf_ctx->filename != NULL)
         SCFree(lf_ctx->filename);
 
-    if (lf_ctx->sensor_name)
-        SCFree(lf_ctx->sensor_name);
-
     if (!lf_ctx->threaded) {
         OutputUnregisterFileRotationFlag(&lf_ctx->rotation_flag);
     }