dnn/dnn_backend_native: Don't use asserts for checks

asserts should not be used instead of ordinary input checks.
Yet the native DNN backend did it: get_input_native() asserted that
the first dimension was one, despite this value coming directly from
the input file without having been sanitized.

Reviewed-by: Guo, Yejun <yejun.guo@intel.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavfilter/dnn/dnn_backend_native.c

@@ -232,6 +232,8 @@ DNNModel *ff_dnn_load_model_native(const char *model_filename, DNNFunctionType f
             oprd->dims[dim] = (int32_t)avio_rl32(model_file_context);
             dnn_size += 4;
         }
+        if (oprd->type == DOT_INPUT && oprd->dims[0] != 1)
+            goto fail;
 
         oprd->isNHWC = 1;
     }