downloadbuffer, benchmarks: fix error leaks in failure code paths

https://bugzilla.gnome.org/show_bug.cgi?id=755019

libs/gst/check/gstcheck.c

@@ -794,6 +794,7 @@ gst_check_abi_list (GstCheckABIStruct list[], gboolean have_abi_sizes)
       if (!g_file_set_contents (fn, s->str, s->len, &err)) {
         g_print ("%s", s->str);
         g_printerr ("\nFailed to write ABI information: %s\n", err->message);
+        g_clear_error (&err);
       } else {
         g_print ("\nWrote ABI information to '%s'.\n", fn);
       }

plugins/elements/gstdownloadbuffer.c

@@ -840,6 +840,7 @@ hit_eos:
 out_flushing:
   {
     GST_DEBUG_OBJECT (dlbuf, "we are flushing");
+    g_clear_error (&error);
     gst_buffer_unmap (buf, &info);
     if (*buffer == NULL)
       gst_buffer_unref (buf);

tests/benchmarks/gstbufferstress.c

@@ -101,6 +101,7 @@ main (gint argc, gchar * argv[])
 
     if (error) {
       printf ("ERROR: g_thread_try_new() %s\n", error->message);
+      g_clear_error (&error);
       exit (-1);
     }
   }

tests/benchmarks/gstclockstress.c

@@ -75,6 +75,7 @@ main (gint argc, gchar * argv[])
 
     if (error) {
       printf ("ERROR: g_thread_try_new() %s\n", error->message);
+      g_clear_error (&error);
       exit (-1);
     }
   }

tests/benchmarks/gstpollstress.c

@@ -165,6 +165,7 @@ main (gint argc, gchar * argv[])
 
     if (error) {
       printf ("ERROR: g_thread_try_new() %s\n", error->message);
+      g_clear_error (&error);
       exit (-1);
     }
   }