sql: return if argument is NULL in sql_xfree()

The sql_xfree() function can take NULL as an argument, and this is not
an error because db->lookaside.pStart > NULL. However, this causes
warnings from static analyzers. Let's add an explicit check to avoid
these warnings.

Part of tarantool/security#120

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring