libkmod: Fix UB for non-existent keys

libkmod: Fix UB for non-existent keys

When running the testsuite with UBSAN and a non-existent info key is
used, the following happens:

TESTSUITE: running test_modinfo_sig_hashalgo, in forked context
TESTSUITE: ERR: 'test_modinfo_sig_hashalgo' [4212] terminated by signal 6 (Aborted)
TESTSUITE: ------
TESTSUITE: running test_modinfo_sig_key, in forked context
TESTSUITE: ERR: 'test_modinfo_sig_key' [4292] terminated by signal 6 (Aborted)
TESTSUITE: ------
TESTSUITE: running test_modinfo_signer, in forked context
TESTSUITE: ERR: 'test_modinfo_signer' [4348] terminated by signal 6 (Aborted)

In CI this happens when disabling module signature handling with
-D openssl=disabled. The UBSAN report is as below:

../dist-unpack/kmod-34/libkmod/libkmod-module.c:1773:2: runtime error: null pointer passed as argument 2, which is declared to never be null
    #0 0x55bb657aa13b  (/__w/kmod/kmod/build/meson-private/dist-build/kmod+0x9f13b) (BuildId: 2c8f9c0ffdbcdf249a3cca29a9d2bc47f706a324)
    #1 0x55bb6580a4be  (/__w/kmod/kmod/build/meson-private/dist-build/kmod+0xff4be) (BuildId: 2c8f9c0ffdbcdf249a3cca29a9d2bc47f706a324)
    #2 0x55bb657cc360  (/__w/kmod/kmod/build/meson-private/dist-build/kmod+0xc1360) (BuildId: 2c8f9c0ffdbcdf249a3cca29a9d2bc47f706a324)
    #3 0x55bb657cdf43  (/__w/kmod/kmod/build/meson-private/dist-build/kmod+0xc2f43) (BuildId: 2c8f9c0ffdbcdf249a3cca29a9d2bc47f706a324)
    #4 0x7f0651bb3487  (/usr/lib/libc.so.6+0x27487) (BuildId: 0b707b217b15b106c25fe51df3724b25848310c0)
    #5 0x7f0651bb354b  (/usr/lib/libc.so.6+0x2754b) (BuildId: 0b707b217b15b106c25fe51df3724b25848310c0)
    #6 0x55bb657aeb14  (/__w/kmod/kmod/build/meson-private/dist-build/kmod+0xa3b14) (BuildId: 2c8f9c0ffdbcdf249a3cca29a9d2bc47f706a324)

Keep the current behavior of appending a 0-length value, just skipping
the pointless memcpy.

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Reviewed-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Link: https://github.com/kmod-project/kmod/pull/300