Crash @WebContentsDelegateEfl::DidFinishLoad

Crash @WebContentsDelegateEfl::DidFinishLoad

When a new window is being opened, it might happen that
about:blank is loaded first, and the actual target URL load
is triggered later.
When that happens, two calls to WebContentsDelegateEfl::DidFinishLoad
are made during the load.
When the first call is made (associated to about:blank) no entry
is added to the NavigationController control, and GetVisibleEntry
returns NULL.

Our implementation wrongly assumed that calling this method would
always return non-null values, and an DCHECK was added.
Patch fixes that assumption by replacing the DCHECK by a null-check.

Note the the signature of the associated NavigationController method
also clearly states that it can return a NULL value:
https://goo.gl/RBiSEu

"
NOTE: This can be nullptr if no entry has committed!
"

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=12867
Change-Id: Ie2f2253c7ec14e15f0b67910023a59d96f71b6f6
Signed-off-by: Antonio Gomes <a1.gomes@samsung.com>