WNM: Fix BSS Termination Duration subelement length validation

WNM: Fix BSS Termination Duration subelement length validation

The length check for the BSS Termination Duration subelement was
accidentally removed and this could result in reading up to 10 bytes
beyond the end of a received frame. The actual read bytes would be
stored locally, but they were not used for anything, so other than
reading beyond the end of an allocated heap memory buffer, this did not
result in any behavior difference or exposure of the bytes.

Credit to OSS-Fuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=14922
Fixes: 093226783dc7 ("WNM: Simplify how candidate subelements are stored")
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>