Commit 4270e9f3243079bb90b6af618ed4d4fd31266412 makes space glyphs in CFF2s zero-width

[Nikolaus Waxweiler]

Testable with e.g. the CFF2 variable font in 
https://github.com/adobe-fonts/source-sans-pro/releases/tag/3.006R in 
Chromium on www.axis-praxis.org.

The cause is this diff:

```
diff --git a/src/psaux/psft.c b/src/psaux/psft.c
index 54be46834..a823ac800 100644
--- a/src/psaux/psft.c
+++ b/src/psaux/psft.c
@@ -313,9 +313,12 @@
    FT_Error error = FT_Err_Ok;
    CF2_Font font;

- FT_Bool is_t1 = decoder->builder.is_t1;
+ FT_Bool is_t1 = decoder->builder.is_t1;


+ if ( !charstring_base || !charstring_len )
+ return FT_ERR( Invalid_File_Format );
+
    FT_ASSERT( decoder &&
               ( is_t1 || decoder->cff ) );
```


Removing the `if` block makes spaces spacy again. I have no idea what 
to do about the returning UBSan warnings though. The check itself seems 
wrong to me. Here's a TTX dump of the space glyph of Source Sans 
Variable:

       <CharString name="space">
       </CharString>