Fix warning: Invalid entry in .debug_gdb_scripts section

Fix warning: Invalid entry in .debug_gdb_scripts section

$ gdb rustc
Reading symbols from rustc...Reading symbols from /usr/lib/debug/usr/bin/rustc.debug...done.
done.
warning: Invalid entry in .debug_gdb_scripts section

/usr/bin/rustc
Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [15] .debug_gdb_scripts PROGBITS        00000000000008ed 0008ed 000022 00 AMS  0   0  1

/usr/lib/debug/usr/bin/rustc.debug
Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [15] .debug_gdb_scripts NOBITS          00000000000008ed 000280 000022 00 AMS  0   0  1

There remains questionable whether bfd_get_section_by_name() should not return
an error for !SEC_LOAD but I haven't investigated that.

gdb/ChangeLog
2017-03-29  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* auto-load.c (auto_load_section_scripts): Check SEC_HAS_CONTENTS.

gdb/testsuite/ChangeLog
2017-03-29  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.python/py-section-script.exp (sepdebug): New testcases.