Switch an assert back to a check.

The assert was supposed to be *added* in fcf25833 but instead replaced
the check.

BUG=465557

Change-Id: I0d3db5038515021e5bdd1ccb9ff08d4f78552621
Reviewed-on: https://boringssl-review.googlesource.com/3850
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

ssl/s3_pkt.c

@@ -201,7 +201,10 @@ int ssl3_read_n(SSL *s, int n, int max, int extend) {
     rb->offset = len + align;
   }
 
-  assert(n <= (int)(rb->len - rb->offset));
+  if (n > (int)(rb->len - rb->offset)) {
+    OPENSSL_PUT_ERROR(SSL, ssl3_read_n, ERR_R_INTERNAL_ERROR);
+    return -1;
+  }
 
   if (!s->read_ahead) {
     /* ignore max parameter */