sumversion: Fix a memory leak in get_src_version()

strsep() modifies its first argument - buf.
An invalid pointer will be passed to the free() function.
Make the pointer passed to free() match the return value of
read_text_file().

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 9413e764 ("kbuild: split the second line of *.mod into *.usyms")
Signed-off-by: Elena Salomatkina <esalomatkina@ispras.ru>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>