bfd: Fix GCC warning when CFLAGS="-Og" is used

This patch initializes the "op" variable in skip_cfa_op() function
of bfd/elf-eh-frame.c to "0" at its declaration point to avoid the
"maybe-uninitialized" warning.

Building binutils on a system with GCC version 13.2.0 and a configure
command that sets the optimization level to "-Og" leads to a build
failure because of a warning being treated as an error:
---------------------------------------------------------------------
$ ./configure CFLAGS="-Og"
$ make
  ...
  CC       elf-eh-frame.lo
  /src/gdb/bfd/elf-eh-frame.c: In function 'skip_cfa_op':
  /src/gdb/bfd/elf-eh-frame.c:354:33: error: 'op' may be used
    uninitialized [-Werror=maybe-uninitialized]
  354 |   switch (op & 0xc0 ? op & 0xc0 : op)
      |           ~~~~~~~~~~~~~~~~~~~~~~^~~~
  /src/gdb/bfd/elf-eh-frame.c:348:12: note: 'op' was declared here
  348 |   bfd_byte op;
      |            ^~
  cc1: all warnings being treated as errors
  ...
---------------------------------------------------------------------

The relevant code snippet related to this warning looks like:
---------------------------------------------------------------------
  static inline bool
  read_byte (bfd_byte **iter, bfd_byte *end, unsigned char *result)
  {
    if (*iter >= end)
      return false;
    *result = *((*iter)++);
    return true;
  }

  static bool
  skip_cfa_op (bfd_byte **iter, bfd_byte *end,...)
  {
    bfd_byte op;

    if (!read_byte (iter, end, &op))
      return false;

    switch (op & 0xc0 ? op & 0xc0 : op)
    ...
  }
---------------------------------------------------------------------

This warning probably happens because "-Og" results in GCC not
inlining the "read_byte()" function. Therefore, GCC treats its
invocation inside "skip_cfa_op()" like a black box and that ends
in the aforementioned warning.

Acknowledgement:
  Lancelot Six -- for coming with the idea behind this fix.
  Jan Beulich  -- for reviewing.

bfd/ChangeLog:
	* elf-eh-frame.c (skip_cfa_op): Initialize the "op" variable.

bfd/elf-eh-frame.c

@@ -345,7 +345,7 @@ next_cie_fde_offset (const struct eh_cie_fde *ent,
 static bool
 skip_cfa_op (bfd_byte **iter, bfd_byte *end, unsigned int encoded_ptr_width)
 {
-  bfd_byte op;
+  bfd_byte op = 0;
   bfd_vma length;
 
   if (!read_byte (iter, end, &op))