[asan] Fix a thinko leading to a crash

[asan] Fix a thinko leading to a crash

It appeared that in my previous patch, a stupid thinko can lead to a
crash when instrumenting some builtin functionsK.  Fixed thus.

Bootstrapped and tested against trunk on x86_64-unknown-linux-gnu.

gcc/
	* asan.c (instrument_builtin_call):  Really put the length of the
	second source argument into src1_len.

gcc/testsuite/

	* c-c++-common/asan/memcmp-2.c: New test.

From-SVN: r196044