posix.cfg: Added minsize attribute to pipe().

cfg/posix.cfg

@@ -1685,7 +1685,9 @@ The function 'mktemp' is considered to be dangerous due to race conditions and s
     <returnValue type="int"/>
     <noreturn>false</noreturn>
     <arg nr="1" direction="out">
+      <not-null/>
       <not-bool/>
+      <minsize type="value" value="2"/>
     </arg>
   </function>
   <!-- int pselect(int nfds, fd_set *restrict readfds,