Assert that pthread mutex initialization succeeded
If pthread mutex initialization fails, the failure will go unnoticed unless
debug assertions are enabled. Any subsequent use of mutex will also silently
fail, since return values from lock & unlock operations are similarly checked
only through debug assertions.
In some implementations the mutex initialization requires a memory
allocation and so it does fail in practice.
Assert that initialization succeeds to ensure that mutex guarantees
mutual exclusion.
Fixes#34966.
b09ef114bb