[PHP] Check if cached file is present before using it

[Frzk]

When deploying a PHP extension, the script downloads a md5 checksum of the extension and compares this value with the one computed from the cache.
If the .md5 file can't be downloaded, it seems that the script considers it's OK to go on and considers that the file in the cache is appropriate.

In my case, the file in the cache wasn't existing, which led to the following trace and error:

+ local checksum_url=https://storage.gra.cloud.ovh.net/v1/AUTH_be65d32d71a6435589a419eac98613f2/scalingo-php-buildpack/scalingo-20/package/amqp-1.11.0.md5
+ local package_url=https://storage.gra.cloud.ovh.net/v1/AUTH_be65d32d71a6435589a419eac98613f2/scalingo-php-buildpack/scalingo-20/package/amqp-1.11.0.tgz
+ sed -u 's/^/       /'
++ curl --fail --retry 3 --retry-delay 2 --connect-timeout 3 --max-time 30 https://storage.gra.cloud.ovh.net/v1/AUTH_be65d32d71a6435589a419eac98613f2/scalingo-php-buildpack/scalingo-20/package/amqp-1.11.0.md5
+ local checksum=
+ local cache_checksum=
+ '[' -f /tmp/cache/package/amqp-1.11.0.md5 ']'
++ dirname amqp-1.11.0
+ mkdir -p /tmp/cache/package/.
+ '[' '' '!=' '' ']'
+ echo 'Checksums match. Fetching from cache.'
+ mkdir -p /app/vendor/amqp
       Checksums match. Fetching from cache.
+ tar xzf /tmp/cache/package/amqp-1.11.0.tgz -C /app/vendor/amqp
tar (child): /tmp/cache/package/amqp-1.11.0.tgz: Cannot open: No such file or directory
tar (child): Error is not recoverable: exiting now
tar: Child returned status 2
tar: Error is not recoverable: exiting now

I suggest to check if the file is actually in the cache before using it.