Fix name() not working in libbacktrace_impls.hpp on mingw-w64

[prbegd]

Fixes #212

Changes

In libbacktrace_impls.hpp:

• libbacktrace_full_callback() will now return 1 when there is valid data. So or case will only call backtrace_syminfo when the data returned from backtrace_pcinfo is invalid.
• Rebase the address (Virtual Address) before passing it to backtrace_syminfo in name_impl. Add read_static_image_base to parse the executable and read the Image Base stored in it.