stats: (hopefully) fix 'Unintended sign extension (SIGN_EXTENSION)'

sign_extension: Suspicious implicit sign extension: "pcr[0]"
with type "unsigned char" (8 bits, unsigned) is promoted in
"pcr[0] * 33554432 + pcr[1] * 131072 + pcr[2] * 512 + pcr[3]
 * 2 + ((pcr[4] & 0x80) >> 7)" to type "int" (32 bits, signed),
then sign-extended to type "unsigned long" (64 bits, unsigned).
If "pcr[0] * 33554432 + pcr[1] * 131072 + pcr[2] * 512 + pcr[3]
 * 2 + ((pcr[4] & 0x80) >> 7)" is greater than 0x7FFFFFFF,
the upper bits of the result will all be 1.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>