PR29846, segmentation fault in objdump.c compare_symbols

Fixes a fuzzed object file problem where plt relocs were manipulated
in such a way that two synthetic symbols were generated at the same
plt location.  Won't occur in real object files.

	PR 29846
	PR 20337
	* objdump.c (compare_symbols): Test symbol flags to exclude
	section and synthetic symbols before attempting to check flavour.