project--read-project-buffer: Fixup default-directory if needed

* lisp/progmodes/project.el (project--read-project-buffer):
Make sure that when the read buffer is new, its default-directory
belongs to the project (bug#62974).
8 jobs for master in 177 minutes and 46 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build Images
passed #66446
build-image-inotify

01:29:31

 
  Normal
passed #66447
test-all-inotify

01:00:48

 
  Platform Images
passed #66448
build-image-eglot

01:27:48

passed #66449
build-image-gnustep

01:40:20

 
  Platforms
passed #66450
test-eglot

00:02:28

passed #66451
test-gnustep

00:02:19

 
  Native Comp Images
passed #66452
build-native-comp-speed2

02:08:26

 
  Native Comp
failed #66453
test-native-comp-speed2

00:49:14

 
Name Stage Failure
failed
test-native-comp-speed2 Native Comp
Uploading artifacts for failed job
Uploading artifacts...
test-native-comp-speed2-4f3dae2b/: found 516 matching files and directories

Uploading artifacts as "archive" to coordinator... 201 Created
id=66453 responseStatus=201 Created token=y7V5jMGr
Uploading artifacts...
test-native-comp-speed2-4f3dae2b/junit-test-report.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... 201 Created
id=66453 responseStatus=201 Created token=y7V5jMGr
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 2