fix: compiler warning UA_clean_errno() invalid conversion error
fix: compiler warning UA_clean_errno() invalid conversion error

If errno is 0 then the function returns "None" string, which
is of type 'const char*'. But the return variable expects
'char*'.
Function strerror returns 'char*' which is ok.

StalderT/open62541,open62541/open62541,JGrothoff/open62541,StalderT/open62541,jpfr/open62541,open62541/open62541,jpfr/open62541,JGrothoff/open62541,StalderT/open62541,jpfr/open62541,open62541/open62541,JGrothoff/open62541,StalderT/open62541,open62541/open62541,jpfr/open62541,JGrothoff/open62541