mbedtls: re-enable an error check

mbedtls: re-enable an error check

While adding support for key blobs, a check and error code update moved
after some logic, resulting in the updated code not checked anymore.

Detected by clang-tidy:
```
lib/vtls/mbedtls.c:768:7: error: Value stored to 'ret' is never read [clang-analyzer-deadcode.DeadStores,-warnings-as-errors]
  768 |       ret = MBEDTLS_ERR_PK_TYPE_MISMATCH;
      |       ^
```
Ref: https://github.com/curl/curl/actions/runs/13953249156/job/39057979349?pr=16764#step:12:178

Regression from 05e0453050def7323fbc8210886f909819edcc5f #7157
Cherry-picked from #16764
Closes #16766