Avoid crash when the font is broken.

The call to FT_New_Face takes the address of the 'face' variable, whose type is a
typedef *something TF_Face;
The value of TF_Face (so a pointer to the properly filled font structure) s then
replaced inside the call of TF_New_Face; but when the latter function fails,
the value of 'face' is not reset and this leads to a crash in the distructor of
TeXFont_PFB.
So properly initialize TF_Face to 0, its address is valid, and the code works.

BUG: 303472
FIXED-IN: 4.8.5
(cherry picked from commit 99c4da2f)