The function WideCharToMultiByte is used at a number of places of the JDK codebase for conversion purposes.
Unfortunately, the function might fail because of various reasons, so the return value must be checked to avoid undefined behavior or even crashes.
see
https://learn.microsoft.com/en-us/windows/win32/api/stringapiset/nf-stringapiset-widechartomultibyte
especially
https://learn.microsoft.com/en-us/windows/win32/api/stringapiset/nf-stringapiset-widechartomultibyte#return-value

At most places in the coding the return values are already checked, but some are missing.