bug#38280: FAIL: test-canonicalize-lgpl with GCC 10

[Martin Liška]

The test fails due to:

./gnulib-tests/test-canonicalize-lgpl
test-canonicalize-lgpl.c:74:14: runtime error: null pointer passed as argument 
1, which is declared to never be null
    #0 0x40169c in main 
(/home/marxin/Programming/sed/gnulib-tests/test-canonicalize-lgpl+0x40169c)
    #1 0x7f76821e8e0a in __libc_start_main ../csu/libc-start.c:308
    #2 0x4011f9 in _start 
(/home/marxin/Programming/sed/gnulib-tests/test-canonicalize-lgpl+0x4011f9

-fsanitize=undefined is used. Problem is that the function has nonnull(1) 
attribute.

One another issue I see with UBSAN:

$ cat testsuite/debug.log
...
debug.pl: test c10_: stderr mismatch, comparing c10_.3 (expected) and c10_.E 
(actual)
*** c10_.3      Tue Nov 19 19:42:55 2019
--- c10_.E      Tue Nov 19 19:42:55 2019
***************
*** 0 ****
--- 1 ----
+ sed/debug.c:322:7: runtime error: null pointer passed as argument 1, which is 
declared to never be null
c11_...
c12...

Thank you,
Martin