avformat/iamf_writer: Don't leak on error when adding ParamDefinition

Fix this by postponing the allocation.
Fixes Coverity issue #1559545.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/iamf_writer.c

@@ -138,10 +138,6 @@ static IAMFParamDefinition *add_param_definition(IAMFContext *iamf, AVIAMFParamD
 
     iamf->param_definitions = tmp;
 
-    param_definition = av_mallocz(sizeof(*param_definition));
-    if (!param_definition)
-        return NULL;
-
     if (audio_element)
         codec_config = iamf->codec_configs[audio_element->codec_config_id];
 
@@ -160,6 +156,10 @@ static IAMFParamDefinition *add_param_definition(IAMFContext *iamf, AVIAMFParamD
             param->constant_subblock_duration = codec_config->nb_samples;
     }
 
+    param_definition = av_mallocz(sizeof(*param_definition));
+    if (!param_definition)
+        return NULL;
+
     param_definition->mode = !!param->duration;
     param_definition->param = param;
     param_definition->audio_element = audio_element;