Initialize "potentially uninitialized" pointers.

[wolfpld]

In certain configurations MSVC may enable some sort of a static analyser during compilation, which marks "unsafe" code constructs. In this case the error messages are:

1>...\zstd\compress\zstd_lazy.c(1399): error C4703: potentially uninitialized local pointer variable 'dmsTagRow' used
1>...\zstd\compress\zstd_lazy.c(1407): error C4703: potentially uninitialized local pointer variable 'dmsRow' used

Initialization and usage of both dmsRow and dmsTagRow is contained within two separate if (dictMode == ZSTD_dictMatchState) sections. It is not possible to enter the second (usage) section without entering the first (initialization), but the analyser fails to see this.

This change silences the "error".

[felixhandte]

This is a reasonable fix for an unfortunate warning.

I worry that zealously fixing these false positives creates room for actually incorrect uses in the future. Initializing these values in this way allows for syntactically correct uses which are nonetheless semantically incorrect. But I don't think there's really anything we can do about that here.