Change stray NULL to nullptr

While change crrev.com/c/718338 was changing NULL to nullptr, change
crrev.com/c/711334 was adding another NULL. I noticed this (eventually)
because I was simultaneously trying to write a change which was very
dependent on the precise value being used for null pointers.

BUG=v8:6928,v8:6921

Change-Id: Ib42cccf90c91c050032cc9e34e44c99fd14ff1bb
Reviewed-on: https://chromium-review.googlesource.com/722619
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48651}

src/parsing/token.h

@@ -147,7 +147,7 @@ namespace internal {
   T(NUMBER, nullptr, 0)                                            \
   T(SMI, nullptr, 0)                                               \
   T(STRING, nullptr, 0)                                            \
-  T(BIGINT, NULL, 0)                                               \
+  T(BIGINT, nullptr, 0)                                            \
                                                                    \
   /* Identifiers (not keywords or future reserved words). */       \
   T(IDENTIFIER, nullptr, 0)                                        \