loader_4th: scan_buffer can leave empty string on stack

When the file processing is done, we will have string with lenght 0 in stack and we will attempt to
allocate 0 bytes.

stand/forth/support.4th

@@ -363,6 +363,7 @@ variable fd
 ;
 
 : line_buffer_resize  ( len -- len )
+  dup 0= if exit then
   >r
   line_buffer .len @ if
     line_buffer .addr @
@@ -376,6 +377,7 @@ variable fd
 ;
     
 : append_to_line_buffer  ( addr len -- )
+  dup 0= if 2drop exit then
   line_buffer strget
   2swap strcat
   line_buffer .len !