fix: fix crash when no default fonts are found (AcademySoftwareFounda…

…tion#4249)

The nullptr at the end of default_font_name was left over from a time
that we iterated by going through array elements until we hit the
nullptr that signalled we had passed the last valid element. At some
point we switched to a range-for, which iterates over every element of
the array without needing an "end sentinel", and we should have
eliminated the nullptr but failed to do so.

Fixes AcademySoftwareFoundation#4248

Signed-off-by: Larry Gritz <lg@larrygritz.com>

src/libOpenImageIO/imagebufalgo_draw.cpp

@@ -883,7 +883,7 @@ static FT_Library ft_library = NULL;
 static bool ft_broken        = false;
 
 static const char* default_font_name[] = { "DroidSans", "cour", "Courier New",
-                                           "FreeMono", nullptr };
+                                           "FreeMono" };