MDEV-12266 fixup: Fix bug in row_ins_sec_index_entry()

row_ins_sec_index_entry(): Compare a pointer to fil_system.sys_space,
not to a numeric constant. This code was recently changed in MDEV-13637,
and the condition was essentially disabled, potentially causing the
change buffer to grow uncontrollably when something is inserted into
a table that has secondary indexes and resides in the system tablespace.

Thanks to Daniel Black for pointing out that clang 7 flagged a warning
for the comparison of a pointer to an integer.

row_import_for_mysql(): Fix a possible compiler warning.

storage/innobase/row/row0import.cc

@@ -3924,7 +3924,7 @@ row_import_for_mysql(
 		true, true, FIL_TYPE_IMPORT, table->space_id,
 		fsp_flags, table->name, filepath, &err);
 
-	ut_ad(!table->space == (err != DB_SUCCESS));
+	ut_ad((table->space == NULL) == (err != DB_SUCCESS));
 	DBUG_EXECUTE_IF("ib_import_open_tablespace_failure",
 			err = DB_TABLESPACE_NOT_FOUND; table->space = NULL;);
 

storage/innobase/row/row0ins.cc

@@ -3309,7 +3309,7 @@ row_ins_sec_index_entry(
 	if (err == DB_FAIL) {
 		mem_heap_empty(heap);
 
-		if (index->table->space == IBUF_SPACE_ID
+		if (index->table->space == fil_system.sys_space
 		    && !(index->type & (DICT_UNIQUE | DICT_SPATIAL))) {
 			ibuf_free_excess_pages();
 		}