munmap: Fail if munmap address range is not mapped

Make sure that the address range passed to munmap() is actually mapped.

Reviewed-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

libc/mman.cc

@@ -161,6 +161,9 @@ int munmap_validate(void *addr, size_t length)
     if (!mmu::is_page_aligned(addr) || length == 0) {
         return EINVAL;
     }
+    if (!mmu::ismapped(addr, length)) {
+        return EINVAL;
+    }
     return 0;
 }