hstore parser incorrectly handles malformed input

It seems that the hstore parser has some odd behavior in the the
handling of certain malformed input constructions:

[db]> select 'a=>,b=>1'::hstore;
hstore
--------------
"a"=>",b=>1"

[db]> select 'a=> ,b=>1'::hstore;
hstore
--------------
"a"=>",b=>1"

[db]> select 'a=>, b=>1'::hstore;
ERROR: Syntax error near 'b' at position 5
LINE 2: select 'a=>, b=>1'::hstore;

In my mind, all of these should have been rejected as erroneous input.
To that end, I have attached a patch which causes all of these inputs
to be rejected as invalid.

-Ryan Kelly