Fix line counting in StringView.cpp

- if fText is an empty string, we would start searching past its end,
  possibly leading to a crash (noticed this in HaikuDepot).
- if fText is NULL, BString would report a size of 0 lines, it makes
  more sense to report a size of 1 line.

src/kits/interface/StringView.cpp

@@ -591,8 +591,8 @@ BStringView::_ValidatePreferredSize()
 		font_height fontHeight;
 		GetFontHeight(&fontHeight);
 
-		int32 lines = 0;
-		char* temp = fText;
+		int32 lines = 1;
+		char* temp = fText ? strchr(fText, '\n') : NULL;
 		while (temp != NULL) {
 			temp = strchr(temp + 1, '\n');
 			lines++;