Fix crash when selecting previous paragraph at buffer begin

Fixes #3489

When there are multiple empty lines between a paragraph and the cursor
(C in the example below), <a-[>p skips over one of them. Prevent the
check for the extra newline from going out of bounds.

```
a paragraph

C	after <a-[>p, the first two lines will be selected
```

src/selectors.cc

@@ -560,7 +560,7 @@ select_paragraph(const Context& context, const Selection& selection,
     BufferIterator first = buffer.iterator_at(selection.cursor());
 
     if (not (flags & ObjectFlags::ToEnd) and first.coord() > BufferCoord{0,1} and
-        is_eol(*(first-1)) and is_eol(*(first-2)))
+        is_eol(*(first-1)) and first-1 != buffer.begin() and is_eol(*(first-2)))
         --first;
     else if ((flags & ObjectFlags::ToEnd) and
              first != buffer.begin() and (first+1) != buffer.end() and

test/regression/3489-crash-on-paragraph-begin/cmd

@@ -0,0 +1 @@
+j<a-[>p

test/regression/3489-crash-on-paragraph-begin/in

@@ -0,0 +1,2 @@
+
+

test/regression/3489-crash-on-paragraph-begin/kak_selection_desc

@@ -0,0 +1 @@
+2.1,1.1