[gdb/symtab] Use early continue in find_pc_sect_compunit_symtab

[gdb/symtab] Use early continue in find_pc_sect_compunit_symtab

Function find_pc_sect_compunit_symtab contains a loop:
...
  for (compunit_symtab *cust : obj_file->compunits ())
    {
      ...
      if (...)
        {
          /* Lots of code.  */
        }
    }
...

Reduce indentation level and improve readability by using early continue.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-10-27  Tom de Vries  <tdevries@suse.de>

	* symtab.c (find_pc_sect_compunit_symtab): Use early continue.