Detect invalid coordinates in selection_from_string

Fixes #1751

src/selection.cc

@@ -498,6 +498,10 @@ Selection selection_from_string(StringView desc)
     BufferCoord cursor{str_to_int({comma+1, dot_cursor}) - 1,
                        str_to_int({dot_cursor+1, desc.end()}) - 1};
 
+    if (anchor.line < 0 or anchor.column < 0 or
+        cursor.line < 0 or cursor.column < 0)
+        throw runtime_error(format("coordinates must be >= 1: '{}'", desc));
+
     return Selection{anchor, cursor};
 }