Fix CU overrun in scan_unit_for_symbols

Fix CU overrun in scan_unit_for_symbols

A customer program had a DWARF CU that consisted of just a CU DIE,
without any children.  In this situation, scan_unit_for_symbols will
try to read past the end of the current CU, and will take use the
first bytes of the next CU as an abbrev, printing an error message.

This patch fixes the bug by changing scan_unit_for_symbols to stop at
the end of the CU rather than the end of the .debug_info section.

bfd/ChangeLog
2019-08-15  Tom Tromey  <tromey@adacore.com>

	* dwarf2.c (scan_unit_for_symbols): Check for end of CU, not end
	of section.