Fix segfault: unwordwrap string w/ 2 start `\n`s

This fixes a segmentation fault caused by an out-of-bounds indexing
caused by an attempt to unwordwrap a string that starts with two
newlines.

The problem here is that in the branch of the function
string_unwordwrap() where `consecutive_newlines == 1`, the function does
not check that the string `result` isn't empty before attempting to
index `result.size()-1`. If `result` is empty, then `result.size()` is
0, and `result.size()-1` becomes -1, and indexing a string at position
-1 is always undefined behavior.

Funnily enough, a similar indexing happens just a few lines down, but
this time, there is a check to make sure that the string isn't empty
first. I'm unsure of how Dav999 forgot that check a few lines earlier.

This situation can happen in practice, with custom level localizations.
I made a level with a filename of testloc.vvvvvv and created a file at
lang/fr/levels/testloc/custom_cutscenes.xml with the following content:

    <?xml version="1.0" encoding="UTF-8"?>
    <cutscenes>
        <cutscene id="test" explanation="">
            <dialogue speaker="cyan" english="This is text..." translation="blarg"/>
        </cutscene>
    </cutscenes>

Then I switched to French, created a script named `test`, and created a
text box that started with two newlines (so in total, the text box must
be at least 3 lines in length). Running the script triggers the segfault
when the text box is created. (Well, technically, on my machine, it
triggers an assertion fail in libstdc++ and aborts, but that's basically
the same thing.)

To fix this while still preserving the exact amount of newlines, if
`result` is empty, we add a newline instead of attempting to index the
string.

desktop_version/src/Font.cpp

@@ -1048,8 +1048,16 @@ std::string string_unwordwrap(const std::string& s)
             }
             else if (consecutive_newlines == 1)
             {
-                // The last character was already a newline, so change it back from the space we thought it should have become.
-                result[result.size()-1] = '\n';
+                if (!result.empty())
+                {
+                    // The last character was already a newline, so change it back from the space we thought it should have become.
+                    result[result.size()-1] = '\n';
+                }
+                else
+                {
+                    // The string starts with two or more newlines, in this case we didn't add the first one at all.
+                    result.append("\n\n");
+                }
             }
             consecutive_newlines++;
         }