Fix for io segfault under gcc11

The segfault only happened with sanitizers off, -O3, and -DRA_DO_CHECK=1, and
only on gcc11; gcc12 and gcc13 work fine, or any -O<3 in gcc11. The error went
away when i removed random combinations of lines in test/io.cc and I couldn't
reduce it. Go figure.

* ra/ply.hh (operator<<): As stated.

ra/ply.hh

@@ -411,6 +411,8 @@ operator<<(std::ostream & o, FormatArray<A> const & fa)
     static_assert(BAD!=size_s<A>(), "Cannot print undefined size expr.");
     auto a = ra::start(fa.a); // [ra35]
     auto sha = shape(a);
+// the following assert fixes a segfault in gcc11.3 test/io.c with -O3 -DRA_DO_CHECK=1.
+    assert(every(ra::start(sha)>=0));
     if (withshape==fa.fmt.shape || (defaultshape==fa.fmt.shape && size_s(a)==ANY)) {
         o << sha << '\n';
     }