libctf: lookup_by_name: do not return success for nonexistent pointer types

The recent work allowing lookups of pointers in child dicts when the
pointed-to type is in the parent dict broke the case where a pointer
type that does not exist at all is looked up: we mistakenly return the
pointed-to type, which is likely not a pointer at all.  This causes
considerable confusion.

Fixed, with a new testcase.

libctf/ChangeLog
2021-01-19  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-lookup.c (ctf_lookup_by_name_internal): Do not return the
	base type if looking up a nonexistent pointer type.
	* testsuite/libctf-regression/pptrtab*: Test it.

libctf/testsuite/libctf-regression/pptrtab-a.c

@@ -1,3 +1,5 @@
 typedef long a_t;
+typedef long b_t;
 
 a_t *a;
+b_t ignore2;