Keep rval refs alive in StringHashTable._unique

Address a Heisenbug caused by `v = get_c_string(<str>repr(val))` potentially pointed to a string that is unreferenced the next time an exception is raised.  (Two exceptions are raised in succession in `pandas/tests/base/test_unique.py test_unique_bad_unicode`.

Signed-off-by: Michael Tiemann <[email protected]>

pandas/_libs/hashtable_class_helper.pxi.in

@@ -1128,6 +1128,7 @@ cdef class StringHashTable(HashTable):
         use_na_value = na_value is not None
 
         # assign pointers and pre-filter out missing (if ignore_na)
+        keep_rval_refs = []
         vecs = <const char **>malloc(n * sizeof(char *))
         for i in range(n):
             val = values[i]
@@ -1144,7 +1145,9 @@ cdef class StringHashTable(HashTable):
                 try:
                     v = get_c_string(<str>val)
                 except UnicodeEncodeError:
-                    v = get_c_string(<str>repr(val))
+                    rval = <str>repr(val)
+                    keep_rval_refs.append(rval)
+                    v = get_c_string(rval)
                 vecs[i] = v
 
         # compute