Fix lz4uncompress

Summary: the library function lz4_uncompress will keep reading input bytes
until it produces the specified number of output bytes (if it doesn't produce
exactly the right number of output bytes, it returns a failure code).

hphp/test/slow/ext_zlib/ext_zlib.php has a test for invalid input. That was
reading beyond the end of the input string, and, given the right garbage
bytes, could actually succeed.

Switch to lz4_decompress_safe, which specifies how long the input string is, and
a maximum number of output bytes.

Reviewed By: @dariorussi

Differential Revision: D1432804