tweak asserts

Author: VSadov

src/coreclr/vm/ceemain.cpp

@@ -1782,7 +1782,7 @@ static void OsAttachThread(void* thread)
 
     if (threadFromCurrentFiber != NULL)
     {
-        _ASSERTE("Multiple threads encountered from a single fiber");
+        _ASSERTE(!"Multiple threads encountered from a single fiber");
         COMPlusThrowWin32();
     }
 
@@ -1812,7 +1812,7 @@ bool OsDetachThread(void* thread)
 
     if (threadFromCurrentFiber != thread)
     {
-        _ASSERTE("Detaching a thread from the wrong fiber");
+        _ASSERTE(!"Detaching a thread from the wrong fiber");
         COMPlusThrowWin32();
     }