udf: Fix NULL pointer dereference in udf_symlink function

In the Linux kernel, the following vulnerability has been resolved:

udf: Fix NULL pointer dereference in udf_symlink function

In function udf_symlink, epos.bh is assigned with the value returned

by udf_tgetblk. The function udf_tgetblk is defined in udf/misc.c

and returns the value of sb_getblk function that could be NULL.

Then, epos.bh is used without any check, causing a possible

NULL pointer dereference when sb_getblk fails.

This fix adds a check to validate the value of epos.bh.