unix: use return value from fread properly

embed/unix/flash.c

@@ -55,20 +55,17 @@ static void flash_sync(void)
     }
 }
 
-static void flash_read(void)
+secbool flash_init(void)
 {
     FILE *f = fopen(FLASH_FILE, "rb");
+    size_t r = 0;
     if (f) {
-        size_t r = fread(flash_buffer, sizeof(flash_buffer), 1, f);
-        (void)r;
+        r = fread(flash_buffer, sizeof(flash_buffer), 1, f);
         fclose(f);
     }
-}
-
-secbool flash_init(void)
-{
-    memset(flash_buffer, 0xFF, sizeof(flash_buffer));
-    flash_read();
+    if (r != 1) {
+        memset(flash_buffer, 0xFF, sizeof(flash_buffer));
+    }
     return sectrue;
 }