AsmPrinter: Check subprogram before using it

Check return of `getDISubprogram()` before using it.  A WIP patch makes
`DIDescriptor` accessors more strict (and would crash on this).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232838 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed