dynamic's valid is too permissive

Working on #5031 for PHPS that relies on pDynamic_6->methods.valid, I found that dynamic's valid is quite permissive.

It does not reject garbage after $HEX$:

$dynamic_6$612f95cf86c4e196f6b627bab3f01b62$HEX$$$$$$$

Is it ok?

I tried to make samples expecting that salt is HEX$$$$$$$ or salt is empty. I could not crack them.

It affects PHPS format allowing it to load such hashes while it should not (its salt is 3 chars exactly).