Fix partial line protection issue

* lisp/org-html.el (org-export-as-html): Do not treat partially
protected lines as if they were fully protected.

Nicolas Goaziou writes:

> Here is a problem when a latex fragment is split across two lines and
> an emphasize follows. The text won't be italicized upon exporting to
> HTML.
>
> =====
> * latex-fragments bug
>
>  Imagine we have a formula starting here $e^{i\pi} +
>  1 = 0$. Now we have a problem with /emphasize/.
> =====
>
> This is because the line starts with a char with 'org-protected
> property and, thus, get caught by the "Protected HTML" (org-html.el
> l. 1216) part of `org-export-as-html'. In others words, the line is
> inserted as-is in the output buffer, before getting any
> transformation.
>
> I'm not sure how it should be done (I don't get yet the usefulness of
> this "Protected HTML" part), but that piece of code may be moved after
> the `org-html-expand' call, as long as every sub-function in
> `org-html-expand' has a check to prevent modifying protected stuff
> (this not yet the case for `org-export-with-emphasize' and
> `org-html-protect' while others seem ok).
>
> But even in this case, every function getting called after that would
> be ignored. So, for example, links would not be inserted.
>
> Couldn't the "Protected HTML" part be removed altogether?

lisp/org-html.el

@@ -1213,7 +1213,11 @@ lang=\"%s\" xml:lang=\"%s\">
 	    (throw 'nextline nil))
 
 	  ;; Protected HTML
-	  (when (get-text-property 0 'org-protected line)
+	  (when (and (get-text-property 0 'org-protected line)
+		     ;; Make sure it is the entire line that is protected
+		     (not (< (or (next-single-property-change
+				  0 'org-protected line) 10000)
+			     (length line))))
 	    (let (par (ind (get-text-property 0 'original-indentation line)))
 	      (when (re-search-backward
 		     "\\(<p>\\)\\([ \t\r\n]*\\)\\=" (- (point) 100) t)