Fix signed integer overflow in data-streamer.c

Fix signed integer overflow in data-streamer.c

Running the testsuite with a -fsanitize=undefined instrumented compiler
shows:
 % gcc -O2 -flto -fno-use-linker-plugin -flto-partition=none testsuite/gcc.dg/torture/pr28045.c
gcc/data-streamer.c:113:45: runtime error: negation of -9223372036854775808 cannot be represented in type 'long int'; cast to an unsigned type to negate this value to itself

The fix is obvious.

From-SVN: r215706