Utils: correctly handle read() errors in ReadRandomBytes()

read() returns -1 on error, so we need to use a signed type.

Test: Booted device with FDE
Change-Id: Ib44247c1cdfd38674a7439209e333823f2fb6cf8

Utils.cpp

@@ -377,7 +377,7 @@ status_t ReadRandomBytes(size_t bytes, char* buf) {
         return -errno;
     }
 
-    size_t n;
+    ssize_t n;
     while ((n = TEMP_FAILURE_RETRY(read(fd, &buf[0], bytes))) > 0) {
         bytes -= n;
         buf += n;