parse-datetime: fix crash or infloop in TZ="" parsing
This was reported in http://bugs.gnu.org/16872
from the coreutils command: date -d 'TZ="""'
The infinite loop for this case was present since the
initial TZ="" parsing support in commit
de95bdc2 29-10-2004.
This was changed to a crash or heap corruption depending
on the platform with commit
2e3e4195 18-01-2010.
* lib/parse-datetime.y (parse_datetime): Break out of the
TZ="" parsing loop once the second significant " is found.
Also skip over any subsequent whitespace to be consistent
with the non TZ= case.
* tests/test-parse-datetime.c: Add test cases for TZ="" parsing.