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.