Add the check_mul_overflow_size_t function from this upstream commit:

commit 2e0bbbfbf95fc9e22692e93658a6fbdd2d4554da
Author: Dennis Wölfing <denniswoelfing@gmx.de>
Date:   Tue May 30 18:26:19 2017 -0300

    Add reallocarray function

diff --git a/malloc/malloc-internal.h b/malloc/malloc-internal.h
index de6103d7e1e06799..dbd801a58eeceabf 100644
--- a/malloc/malloc-internal.h
+++ b/malloc/malloc-internal.h
@@ -81,5 +81,24 @@ void __malloc_fork_unlock_parent (void) internal_function attribute_hidden;
 /* Called in the child process after a fork.  */
 void __malloc_fork_unlock_child (void) internal_function attribute_hidden;
 
+/* Set *RESULT to LEFT * RIGHT.  Return true if the multiplication
+   overflowed.  */
+static inline bool
+check_mul_overflow_size_t (size_t left, size_t right, size_t *result)
+{
+#if __GNUC__ >= 5
+  return __builtin_mul_overflow (left, right, result);
+#else
+  /* size_t is unsigned so the behavior on overflow is defined.  */
+  *result = left * right;
+  size_t half_size_t = ((size_t) 1) << (8 * sizeof (size_t) / 2);
+  if (__glibc_unlikely ((left | right) >= half_size_t))
+    {
+      if (__glibc_unlikely (right != 0 && *result / right != left))
+        return true;
+    }
+  return false;
+#endif
+}
 
 #endif /* _MALLOC_INTERNAL_H */