video: Fix false positives in driver name comparison

Without this change, driver names don't get matched correctly;
for example "x" can get matched with "x11" since it only checks
whether the string matches up to the length of the requested
driver name.

src/video/SDL_video.c

@@ -505,7 +505,8 @@ SDL_VideoInit(const char *driver_name)
                                                                      : SDL_strlen(driver_attempt);
 
             for (i = 0; bootstrap[i]; ++i) {
-                if (SDL_strncasecmp(bootstrap[i]->name, driver_attempt, driver_attempt_len) == 0) {
+                if ((driver_attempt_len == SDL_strlen(bootstrap[i]->name)) &&
+                    (SDL_strncasecmp(bootstrap[i]->name, driver_attempt, driver_attempt_len) == 0)) {
                     video = bootstrap[i]->create(index);
                     break;
                 }