Fixes to build with MSVC with /W4

[cwoffenden]

Updating to 1.5.0 no longer builds with MSVC's warning level 4 due to "potentially uninitialized" local vars. It's a false positive, since each use is wrapped in a test for if (dictMode == ZSTD_dictMatchState). The quick fix is to globally ignore the warnings, silencing these:

error C2220: the following warning is treated as an error
warning C4701: potentially uninitialized local variable 'nextSeqStore' used
warning C4701: potentially uninitialized local variable 'ddsIdx' used
warning C4701: potentially uninitialized local variable 'ddsExtraAttempts' used
warning C4701: potentially uninitialized local variable 'dmsTag' used
warning C4701: potentially uninitialized local variable 'dmsTagRow' used
error C4703: potentially uninitialized local pointer variable 'dmsTagRow' used
warning C4701: potentially uninitialized local variable 'dmsRow' used
error C4703: potentially uninitialized local pointer variable 'dmsRow' used

It's not a comfortable warning to globally silence so the alternative would be to locally silence them (which is preferable to setting them to anything just to make the compiler happy).

[cwoffenden]

This is doing the same as #2654 just in a different way.

[felixhandte]

I've just merged the other PR. So I'm going to close this in the expectation that the underlying issue is fixed.

Thanks for the contribution though!