Fix implicit conversion loses integer precision

Fix for the following warning:
Implicit conversion loses integer precision: 'unsigned long' to 'int'

Change-Id: I4d4453401eca1136b0cbf21b630893f242d7bb55

av1/decoder/accounting.c

diff --git a/av1/decoder/accounting.c b/av1/decoder/accounting.c
index ba243c9..fdbb9a5 100644
--- a/av1/decoder/accounting.c
+++ b/av1/decoder/accounting.c
@@ -31,7 +31,7 @@
 /* Dictionary lookup based on an open-addressing hash table. */
 int aom_accounting_dictionary_lookup(Accounting *accounting, const char *str) {
   int hash;
-  int len;
+  size_t len;
   AccountingDictionary *dictionary;
   dictionary = &accounting->syms.dictionary;
   hash = aom_accounting_hash(str);