make sure multipart parsing has enough bytes to compare against

basis/mime/multipart/multipart.factor

@@ -42,7 +42,7 @@ ERROR: end-of-stream multipart ;
     [ t >>end-of-stream? ] if* ;
 
 : maybe-fill-bytes ( multipart -- multipart )
-    dup bytes>> [ fill-bytes ] unless  ;
+    dup bytes>> length 256 < [ fill-bytes ] when ;
 
 : split-bytes ( bytes separator -- leftover-bytes safe-to-dump )
     dupd [ length ] bi@ 1- - short cut-slice swap ;
@@ -65,6 +65,7 @@ ERROR: end-of-stream multipart ;
     [ dump-until-separator ] with-string-writer ;
 
 : read-header ( multipart -- multipart )
+    maybe-fill-bytes
     dup bytes>> "--\r\n" sequence= [
         t >>end-of-stream?
     ] [