Prevent unaligned memory read in parseMinidumpString

Summary:
It's possible to hit an unaligned memory read when reading `source_length` as the `data` array is only aligned with 2 bytes (it's actually a UTF16 array). This patch memcpy's `source_length` into a local variable to prevent this:

```
MinidumpTypes.cpp:49:23: runtime error: load of misaligned address 0x7f0f4792692a for type 'const uint32_t' (aka 'const unsigned int'), which requires 4 byte alignment
```

Reviewers: dvlahovski, zturner, davide

Reviewed By: davide

Subscribers: davide, lldb-commits

Differential Revision: https://reviews.llvm.org/D42348

llvm-svn: 323181