Refuse to push a non-regular file server

If SCRCPY_SERVER_PATH points to a directory, then a directory will be
pushed to /data/local/tmp/scrcpy-server.jar.

When executing it, app_process will just abort and leave the directory
on the device, causing scrcpy to always fail.

To avoid the problem, check that the server is a regular file before
pushing it.

Closes #956 <https://github.com/Genymobile/scrcpy/issues/956>

app/src/server.c

@@ -67,7 +67,12 @@ get_server_path(void) {
 
 static bool
 push_server(const char *serial) {
-    process_t process = adb_push(serial, get_server_path(), DEVICE_SERVER_PATH);
+    const char *server_path = get_server_path();
+    if (!is_regular_file(server_path)) {
+        LOGE("'%s' does not exist or is not a regular file\n", server_path);
+        return false;
+    }
+    process_t process = adb_push(serial, server_path, DEVICE_SERVER_PATH);
     return process_check_success(process, "adb push");
 }