vp9: Fix multiplication overflow

Fix UBSan error reported from aosp Cuttlefish device:
/vp9/encoder/vp9_ratectrl.c:238:33: unsigned integer overflow:
2500000 * 1800 cannot be represented in type 'unsigned int'

...by casting the operand and the result of multiplication
to 64bit integer.

Test: vp9 webrtc streaming with Cuttlefish
Change-Id: Id5bb3d4071a96179caffae0829d3cc4e48c7614b
1 file changed