libdw: Detect bad DWARF in store_implicit_value.

libdw: Detect bad DWARF in store_implicit_value.

The afl fuzzer running against the varlocs test detected we didn't report
the value block of a DW_OP_implicit_value consistently when the DWARF was
bad. Although this doesn't cause a crash it might result in consumers
using dwarf_getlocation_implicit_value seeing an inconsistent block length
value. To fix this detect and report bad DWARF data earlier.

Signed-off-by: Mark Wielaard <mark@klomp.org>