Fixes #1039.

* src/base/ftstroke.c (ft_stroker_inside, ft_stroker_outside):
Initialize `sigma`.

* src/sdf/ftsdf.c (sdf_generate_with_overlaps): Exit immediately if
function arguments are invalid.
* src/sdf/ftsdfrend.c (sdf_property_set) <"overlaps">: Fix cast.

* src/sfnt/sfwoff2.c (woff2_decompress)
[!FT_CONFIG_OPTION_USE_BROTLI]: Use `FT_UNUSED`.

* src/truetype/ttgxvar.c (TT_Get_MM_Var): Initialize `fvar_head`.