Fixed USB serial zero length string issue in hardwareserial_cdcacm.c
Fixed USB serial zero length string issue in hardwareserial_cdcacm.c


Former-commit-id: 0b4998e557243efe22a6919183b2e513f86f0dd4

ricklon/chipKIT-core,EmbeddedMan/chipKIT-core,UECIDE/chipKIT-core,pontech/chipKIT-core,ricklon/chipKIT-core,UECIDE/chipKIT-core,chipKIT32/chipkit-core,EmbeddedMan/chipKIT-core,pontech/chipKIT-core,adamwolf/chipKIT-core,majenkotech/chipKIT-core,pontech/chipKIT-core,pontech/chipKIT-core,adamwolf/chipKIT-core,adamwolf/chipKIT-core,majenkotech/chipKIT-core,EmbeddedMan/chipKIT-core,majenkotech/chipKIT-core,EmbeddedMan/chipKIT-core,chipKIT32/chipkit-core,pontech/chipKIT-core,adamwolf/chipKIT-core,EmbeddedMan/chipKIT-core,EmbeddedMan/chipKIT-core,ricklon/chipKIT-core,adamwolf/chipKIT-core,chipKIT32/chipkit-core,majenkotech/chipKIT-core,pontech/chipKIT-core,chipKIT32/chipkit-core,chipKIT32/chipkit-core,pontech/chipKIT-core,ricklon/chipKIT-core,majenkotech/chipKIT-core,adamwolf/chipKIT-core,majenkotech/chipKIT-core,UECIDE/chipKIT-core,adamwolf/chipKIT-core,ricklon/chipKIT-core,UECIDE/chipKIT-core,majenkotech/chipKIT-core,ricklon/chipKIT-core,UECIDE/chipKIT-core,EmbeddedMan/chipKIT-core