file_data smtp: fix minor coverity warning

CID 1298891:  Null pointer dereferences  (REVERSE_INULL)
Null-checking "curr_file" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.

src/detect-engine-filedata-smtp.c

@@ -144,7 +144,6 @@ static uint8_t *DetectEngineSMTPGetBufferForTX(uint64_t tx_id,
         goto end;
     }
 
-    if (curr_file != NULL) {
     int first = 1;
     curr_chunk = curr_file->chunks_head;
     while (curr_chunk != NULL) {
@@ -189,7 +188,6 @@ static uint8_t *DetectEngineSMTPGetBufferForTX(uint64_t tx_id,
 
     /* updat inspected tracker */
     curr_file->content_inspected = curr_file->chunks_tail->stream_offset + curr_file->chunks_tail->len;
-    }
 
     buffer = det_ctx->smtp[index].buffer;
     *buffer_len = det_ctx->smtp[index].buffer_len;