Avoid overwriting string memory in the PDO::mysql driver

Summary: std::strcpy() attempts to overwrite `host` which
at that point is pointing to either user-supplied data,
or worse read-only options memory if `host` hasn't been set.

Make a copy for use within the function.

Closes #4254

Reviewed By: @fredemmott

Differential Revision: D1684162

Signature: t1:1684162:1417460400:48a508bf76edb65de5de58ee8981bb6d507d50a4