Add a call to aom_clear_system_state()

Just before the use of av1_find_qindex() in
find_fp_qindex().

BUG=aomedia:2697

Change-Id: Ia18c1eb8fede45ec04c9f3e1bcdba51aa9c85c6b

av1/encoder/firstpass.c

diff --git a/av1/encoder/firstpass.c b/av1/encoder/firstpass.c
index fdc0465..d50c492 100644
--- a/av1/encoder/firstpass.c
+++ b/av1/encoder/firstpass.c
@@ -267,6 +267,7 @@
 }
 
 static int find_fp_qindex(aom_bit_depth_t bit_depth) {
+  aom_clear_system_state();
   return av1_find_qindex(FIRST_PASS_Q, bit_depth, 0, QINDEX_RANGE - 1);
 }