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.