misc: pci_endpoint_test: Validate BAR index in doorbell test

misc: pci_endpoint_test: Validate BAR index in doorbell test

pci_endpoint_test_doorbell() reads the BAR number directly from an endpoint
test register and uses it as an index into test->bar[].  Add a defensive
bounds check before the dereference: positive values >= PCI_STD_NUM_BARS
are out of range, and NO_BAR (-1) as a negative signed value would slip
past an upper-bound-only check.

Signed-off-by: Carlos Bilbao (Lambda) <carlos.bilbao@kernel.org>
[mani: changed errno to -ERANGE]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260410230300.135631-2-carlos.bilbao@kernel.org