tests: varlocs workaround format-overflow errors

tests: varlocs workaround format-overflow errors

In function ‘printf’,
    inlined from ‘handle_attr’ at varlocs.c:932:3:
error: ‘%s’ directive argument is null [-Werror=format-overflow=]

The warning is technically correct. A %s argument should not be
NULL. Although in practice all implementations will print it as
"(null)". Workaround this by simply changing the dwarf string
functions to return an "<unknown>" string. The test is for the correct
names, either "(null)" or "<unknown>" would make it fail (also remove
a now unnecessary assert, the switch statement will check for unknown
opcodes anyway).

Signed-off-by: Mark Wielaard <mark@klomp.org>
[sw0312.kim: partially pick upstream commit fd2cadbe4749f43551f7df90ee41837d83a6fbc4
   to fix build warning except src/ChangeLog]
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: I52273c671e9ecea250135096629e708163c5aa16