Fix a use of undefined memory in pecoff.c

In rust-lang/rust#39468 it was discovered that this could cause a crash in
libbacktrace due to freeing uninitialized memory, and this specific instance was
fixed in rust-lang/rust#39509

pecoff.c

@@ -609,7 +609,7 @@ coff_add (struct backtrace_state *state, int descriptor,
  int str_view_valid;
  size_t str_size;
  off_t str_off;
- struct backtrace_view syms_view;
+ struct backtrace_view syms_view = {0};
  off_t syms_off;
  size_t syms_size;
  int syms_view_valid;