libdwfl: Clear symtab result on error before using find_dynsym fallback.

libdwfl: Clear symtab result on error before using find_dynsym fallback.

find_dynsym could succeed after find_symtab failed but had already set up
symdata, syments and first_global. find_dynsym would not set or clear
first_global since all syms in dynsym are global. Causing lots of failing
lookups when calling __libdwfl_addrsym if first_global was some huge
bogus value.

Reported-by: Hanno Böck <hanno@hboeck.de>
Signed-off-by: Mark Wielaard <mjw@redhat.com>