fuzz: fix size type

Fix silent overflow in the fuzzer. 255 is clearly << 65536 so that means
we are not able to exercise the full buffer. But upon further inspection
its clear that numbers are a minimum truncated if not rolling over based
on the code in the size_t to uint8_t conversions.

This was validated by converting the type temporarily to signed and
observing the fuzzer using a negative index.

Signed-off-by: Curtis Malainey <cujomalainey@chromium.org>

Author: cujomalainey

src/platform/posix/ipc.c

@@ -30,7 +30,7 @@ extern uint8_t *posix_fuzz_buf, posix_fuzz_sz;
 // thinking/experimentation about how much fuzzing we want to do at a
 // time...
 static uint8_t fuzz_in[65536];
-static uint8_t fuzz_in_sz;
+static size_t fuzz_in_sz;
 
 // The protocol here is super simple: the first byte is a message size
 // in units of 16 bits (the buffer maximum defaults to 384 bytes, and