Fix memory leak in sqlexec()

Callers are supposed to free the error strings themselves:
https://www.sqlite.org/capi3ref.html#sqlite3_exec

Found by Coverity.

lib/backend/sqlite.c

@@ -233,6 +233,7 @@ static int sqlexec(sqlite3 *sdb, const char *fmt, ...)
 	rpmlog(RPMLOG_DEBUG, "%s: %d\n", cmd, rc);
 
     sqlite3_free(cmd);
+    sqlite3_free(err);
 
     return rc ? RPMRC_FAIL : RPMRC_OK;
 }