modpost: fix null pointer dereference

If the find_fromsym() call fails and returns NULL, the warn() call
will dereference this NULL pointer and cause the program to crash.

This happened when I tried to build with "test_user_copy" module.
With this fix, it prints lots of warnings like this:

 WARNING: modpost: lib/test_user_copy: section mismatch in reference: (unknown)+0x4 (section: .text.fixup) -> (unknown) (section: .init.text)

masahiroy@kernel.org:
 The issue is reproduced with ARCH=arm allnoconfig + CONFIG_MODULES=y +
 CONFIG_RUNTIME_TESTING_MENU=y + CONFIG_TEST_USER_COPY=m
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/mod/modpost.c

@@ -1050,7 +1050,9 @@ static void default_mismatch_handler(const char *modname, struct elf_info *elf,
 	sec_mismatch_count++;
 
 	warn("%s: section mismatch in reference: %s+0x%x (section: %s) -> %s (section: %s)\n",
-	     modname, fromsym, (unsigned int)(faddr - from->st_value), fromsec, tosym, tosec);
+	     modname, fromsym,
+	     (unsigned int)(faddr - (from ? from->st_value : 0)),
+	     fromsec, tosym, tosec);
 
 	if (mismatch->mismatch == EXTABLE_TO_NON_TEXT) {
 		if (match(tosec, mismatch->bad_tosec))