Fix minor memory leaks in pg_dump.

I found these by running pg_dump under "valgrind --leak-check=full".

The changes in flagInhIndexes() and getIndexes() replace allocation of
an array of which we use only some elements by individual allocations
of just the actually-needed objects.  The previous coding wasted some
memory, but more importantly it confused valgrind's leak tracking.

collectComments() and collectSecLabels() remain major blots on
the valgrind report, because they don't PQclear their query
results, in order to avoid a lot of strdup's.  That's a dubious
tradeoff, but I'll leave it alone here; an upcoming patch will
modify those functions enough to justify changing the tradeoff.

src/bin/pg_dump/pg_backup_archiver.c

@@ -3374,6 +3374,8 @@ _selectTableAccessMethod(ArchiveHandle *AH, const char *tableam)
 
 	destroyPQExpBuffer(cmd);
 
+	if (AH->currTableAm)
+		free(AH->currTableAm);
 	AH->currTableAm = pg_strdup(want);
 }