CamelPOP3Folder: Fix a potential g_object_unref(NULL) call

If all the branch conditions in this function evaluate to FALSE, the
stream is never set, and hence g_object_unref(NULL) is called on exiting
the function.

Found by scan-build.

https://bugzilla.gnome.org/719533
EBookBackendCache: Remove an unnecessary NULL check.

A newly-created EContact is never NULL, so the check is redundant.
It was confusing the static analyser.

https://bugzilla.gnome.org/719533
libecal: Refactor system timezone key file parsing code

Previously, the code could call g_strstrip(NULL) if a line started with
a quotation mark but didn’t end with one, e.g.:
    MyKey="some-unbalanced-value

The code now ignores unbalanced quotation marks, returning the odd one
in the value rather than crashing (or stripping it).

Found by scan-build.

https://bugzilla.gnome.org/719533
libedata-cal: Fix a potential g_strstrip(NULL) call

If the calendar contained any attendees _not_ prefixed with ‘mailto:’,
the text variable would be NULL, and would cause an assertion failure in
g_strstrip().

Found by scan-build.

https://bugzilla.gnome.org/719533
tests: Fix potential g_object_unref(NULL) calls in libebook tests

Found by scan-build.

https://bugzilla.gnome.org/719533