modpost: do not make find_tosym() return NULL

As mentioned in commit 3975865 ("modpost: Add '.ltext' and
'.ltext.*' to TEXT_SECTIONS"), modpost can result in a segmentation
fault due to a NULL pointer dereference in default_mismatch_handler().

find_tosym() can return the original symbol pointer instead of NULL
if a better one is not found.

This fixes the reported segmentation fault.

Fixes: a23e758 ("modpost: unify 'sym' and 'to' in default_mismatch_handler()")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/mod/modpost.c

@@ -1007,6 +1007,8 @@ static Elf_Sym *find_fromsym(struct elf_info *elf, Elf_Addr addr,
 
 static Elf_Sym *find_tosym(struct elf_info *elf, Elf_Addr addr, Elf_Sym *sym)
 {
+	Elf_Sym *new_sym;
+
 	/* If the supplied symbol has a valid name, return it */
 	if (is_valid_name(elf, sym))
 		return sym;
@@ -1015,8 +1017,9 @@ static Elf_Sym *find_tosym(struct elf_info *elf, Elf_Addr addr, Elf_Sym *sym)
 	 * Strive to find a better symbol name, but the resulting name may not
 	 * match the symbol referenced in the original code.
 	 */
-	return symsearch_find_nearest(elf, addr, get_secindex(elf, sym),
-				      true, 20);
+	new_sym = symsearch_find_nearest(elf, addr, get_secindex(elf, sym),
+					 true, 20);
+	return new_sym ? new_sym : sym;
 }
 
 static bool is_executable_section(struct elf_info *elf, unsigned int secndx)