logging: several memory allocation belong to tall_log_ctx context

Several talloc_zero in logging use NULL context, use tall_log_ctx
instead.
diff --git a/src/logging.c b/src/logging.c
index 7b1ab13..7fabfcb 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -448,7 +448,7 @@
 		size += strlen(loglevel_strs[i].str) + 1;
 
 	rem = size;
-	str = talloc_zero_size(NULL, size);
+	str = talloc_zero_size(tall_log_ctx, size);
 	if (!str)
 		return NULL;
 
@@ -519,7 +519,7 @@
 
 	size += strlen("Global setting for all subsystems") + 1;
 	rem = size;
-	str = talloc_zero_size(NULL, size);
+	str = talloc_zero_size(tall_log_ctx, size);
 	if (!str)
 		return NULL;