http: clear header pointer on realloc failure

Fixes:

detect-engine-hhd.c:188:5: warning: Use of memory after it is freed
    return headers_buffer;

src/detect-engine-hhd.c

@@ -158,8 +158,10 @@ static uint8_t *DetectEngineHHDGetBufferForTX(htp_tx_t *tx, uint64_t tx_id,
         /* the extra 4 bytes if for ": " and "\r\n" */
         uint8_t *new_headers_buffer = SCRealloc(headers_buffer, headers_buffer_len + size1 + size2 + 4);
         if (unlikely(new_headers_buffer == NULL)) {
-            if (headers_buffer != NULL)
+            if (headers_buffer != NULL) {
                 SCFree(headers_buffer);
+                headers_buffer = NULL;
+            }
             det_ctx->hhd_buffers[index] = NULL;
             det_ctx->hhd_buffers_len[index] = 0;
             goto end;