CVE-2019-15550: Out of bounds read in simd-json

The affected version of this crate did not guard against accessing memory beyond the range of its input data. A pointer cast to read the data into a 256-bit register could lead to a segmentation fault when the end plus the 32 bytes (256 bit) read would overlap into the next page during string parsing. This allows an attacker to eventually crash a service. The flaw was corrected by using a padding buffer for the last read from the input. So that we are we never read over the boundary of the input data.