std.cfg: Added a valid-tag for read streamsize.

Author: orbitcowboy

cfg/std.cfg

@@ -4064,6 +4064,7 @@ The obsolete function 'gets' is called. With 'gets' you'll get a buffer overrun
     </arg>
     <arg nr="2">
       <not-uninit/>
+      <valid>0:</valid>
     </arg>
   </function>
   <!-- Not part of standard, but widely supported by runtime libraries. -->