valgrind: ignore SSE-based strlen invalid reads

Some versions of strlen use SSE to speed up the calculation and load 4
bytes at a time, even if it means reading past the end of the
allocated memory. This read is safe and when the strlen function is
inlined, it is not replaced by valgrind, which reports a
false-possitive.

Tell valgrind to ignore this particular error, as the read is, in
fact, safe. Current upstream-released version 3.6.1 is affected. Some
distributions have this fixed in their latest versions.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/valgrind/default.supp

@@ -43,3 +43,9 @@
 	fun:write_buffer
 	fun:write_loose_object
 }
+
+{
+	ignore-sse-strlen-invalid-read-size
+	Memcheck:Addr4
+	fun:copy_ref
+}