client.c: Added more server responses checks

check_version() verifies more server responses:
- If versionFromServer is NULL;
- If the server read an invalid version file (versionFromServer contains
"Invalid version");
- And if the server could not find the version file (versionFromServer
contains "Version file not found").

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Acked-by: Diego Giovane Pasqualin <dgp06@c3sl.ufpr.br>
Signed-off-by: Danilo K. S. Yorinori <danilok@c3sl.ufpr.br>