Fix warnings building with MSVC on Windows

[hughbe]

Old build log - warnings now gone. We know what we're doing, and lengths can never be negative anyways!

[25/65] Building C object src\CMakeFiles\libcmark_static.dir\latex.c.obj
C:\Users\hughb\Documents\GitHub\my-swift\cmark\src\latex.c(176): warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
[37/65] Building C object src\CMakeFiles\cmark.dir\latex.c.obj
C:\Users\hughb\Documents\GitHub\my-swift\cmark\src\latex.c(176): warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
[49/65] Building C object src\CMakeFiles\libcmark.dir\latex.c.obj
C:\Users\hughb\Documents\GitHub\my-swift\cmark\src\latex.c(176): warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data
[61/65] Linking C shared library src\cmark.dll
   Creating library src\cmark.lib and object src\cmark.exp
   Creating library src\cmark.lib and object src\cmark.exp
[65/65] Linking CXX executable api_test\api_test.exe

@jgm @kivikakk

[jgm]

Thanks!