possible driver path truncation

[ceyusa]

When compiling with gcc 8, it complains about a possible string truncation when constructing the driver path.

../subprojects/libva/va/va.c: In function ‘va_openDriver’:
../subprojects/libva/va/va.c:381:9: warning: ‘strncat’ specified bound 1 equals source length [-Wstringop-overflow=]                                                                                              
         strncat( driver_path, "/", strlen("/") );
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../subprojects/libva/va/va.c:383:9: warning: ‘strncat’ specified bound 13 equals source length [-Wstringop-overflow=]                                                                                             
         strncat( driver_path, DRIVER_EXTENSION, strlen(DRIVER_EXTENSION) );
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../subprojects/libva/va/va.c:380:9: warning: ‘strncpy’ specified bound depends on the length of the source argument [-Wstringop-overflow=]                                                                        
         strncpy( driver_path, driver_dir, strlen(driver_dir) + 1);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../subprojects/libva/va/va.c:380:43: note: length computed here
         strncpy( driver_path, driver_dir, strlen(driver_dir) + 1);
                                           ^~~~~~~~~~~~~~~~~~
../subprojects/libva/va/va.c:382:9: warning: ‘strncat’ specified bound depends on the length of the source argument [-Wstringop-overflow=]                                                                        
         strncat( driver_path, driver_name, strlen(driver_name) );
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~