Fix a regression in CJBig2_Context::HuffmanAssignCode().

An assignment statement accidentally got removed in
https://pdfium-review.googlesource.com/c/pdfium/+/31538.

BUG=chromium:880920

Change-Id: Iaff20bd2484cd8bf6dc6a13cbf444cefbd605b25
Reviewed-on: https://pdfium-review.googlesource.com/42250
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

core/fxcodec/jbig2/JBig2_Context.cpp

diff --git a/core/fxcodec/jbig2/JBig2_Context.cpp b/core/fxcodec/jbig2/JBig2_Context.cpp
index d019cc1..4656bfa 100644
--- a/core/fxcodec/jbig2/JBig2_Context.cpp
+++ b/core/fxcodec/jbig2/JBig2_Context.cpp
@@ -1262,6 +1262,7 @@
   std::vector<int> FIRSTCODE(LENMAX + 1);
   for (uint32_t i = 0; i < NTEMP; ++i)
     ++LENCOUNT[SBSYMCODES[i].codelen];
+  LENCOUNT[0] = 0;
 
   for (int i = 1; i <= LENMAX; ++i) {
     pdfium::base::CheckedNumeric<int> shifted = FIRSTCODE[i - 1];