Fix lvfontio memory allocation with fixed width fonts

There was a branch of the loading that failed to set max_slots.
This led to unpredicatable allocation failures.

Fixes micropython#10560

Author: tannewt

shared-module/lvfontio/OnDiskFont.c

@@ -253,9 +253,10 @@ static bool load_font_header(lvfontio_ondiskfont_t *self, FIL *file, size_t *max
                         *max_slots = advance_count[1] * 2 + advance_count[0];
                     }
                 }
+            } else {
+                *max_slots = advance_count[0] + advance_count[1];
             }
 
-
             found_glyf = true;
         }