[FIX] getUserShareDifficulty must return 0 on NULL
[FIX] getUserShareDifficulty must return 0 on NULL

BlueDragon747/php-mpos,CoinHashMe/MPOS-SSO,MPOS/php-mpos,ychaim/php-mpos,machinecoin-project/php-mpos,shreeneve/test,ychaim/php-mpos,UNOMP/chunky-mpos,CoinHashMe/MPOS-SSO,sasselin/xcoin-mpos,studio666/php-mpos,mmitech/php-mpos,MPOS/php-mpos,xisi/php-mpos,mmitech/php-mpos,sasselin/xcoin-mpos,UNOMP/chunky-mpos,evgenyponomarev/pool,machinecoin-project/php-mpos,sasselin/xcoin-mpos,bankonme/php-mpos,mculp/chunky-mpos,shreeneve/test,evgenyponomarev/pool,CoinHashMe/MPOS-SSO,BlueDragon747/php-mpos,nimblecoin/web,evgenyponomarev/pool,bankonme/php-mpos,xisi/php-mpos,mculp/chunky-mpos,sixxkilur/php-mpos,sixxkilur/php-mpos,nimblecoin/web,bankonme/php-mpos,studio666/php-mpos,UNOMP/chunky-mpos,MPOS/php-mpos,mmitech/php-mpos,shreeneve/test,nimblecoin/web,mculp/chunky-mpos,CoinHashMe/MPOS-SSO,xisi/php-mpos,mculp/chunky-mpos,UNOMP/chunky-mpos,machinecoin-project/php-mpos,ychaim/php-mpos,studio666/php-mpos,BlueDragon747/php-mpos,CoinHashMe/MPOS-SSO,sixxkilur/php-mpos