pthread_cond_wait may return spuriously

The man page states: "In general, whenever a condition wait returns, the thread
has to re-evaluate the predicate associated with the condition wait to determine
whether it can safely proceed, should wait again, or should declare a timeout. A
return from the wait does not imply that the associated predicate is either true
or false."

Author: tautschnig

src/ansi-c/library/pthread_lib.c

@@ -636,8 +636,8 @@ inline int pthread_cond_wait(
   #endif
 
   __CPROVER_atomic_begin();
-  __CPROVER_assume(*((unsigned *)cond));
-  (*((unsigned *)cond))--;
+  if(*((unsigned *)cond))
+    (*((unsigned *)cond))--;
   __CPROVER_atomic_end();
 
   return 0; // we never fail