user-exec.c: Don't assert on segfaults for non-valid addresses

h2g() will assert if passed an address that's not a valid guest address,
so handle_cpu_signal() needs to check before passing "data address
which caused a segfault" to it, since for a misbehaving guest
that could be anything. If the address isn't a valid guest address
then we can simply skip the attempt to unprotect a guest page
which was made read-only to catch self-modifying code.

This assertion probably fires more readily now than it used to
do because of recent changes to default to reserving guest address
space.
Acked-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

user-exec.c

@@ -97,7 +97,8 @@ static inline int handle_cpu_signal(uintptr_t pc, unsigned long address,
                 pc, address, is_write, *(unsigned long *)old_set);
 #endif
     /* XXX: locking issue */
-    if (is_write && page_unprotect(h2g(address), pc, puc)) {
+    if (is_write && h2g_valid(address)
+        && page_unprotect(h2g(address), pc, puc)) {
         return 1;
     }