starmath: Avoid creating SmFontNode with a default token

when an unexpected token is found.

Change-Id: I1a11c08ce54c55bb32289498fbb26af019396b83
Reviewed-on: https://gerrit.libreoffice.org/34918Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Takeshi Abe <tabe@fixedpoint.jp>

starmath/source/parse.cxx

@@ -1802,7 +1802,10 @@ void SmParser::DoColor()
             NextToken();
         }
         else
+        {
             Error(SmParseError::ColorExpected);
+            return;
+        }
     } while (m_aCurToken.eType == TCOLOR);
 
     m_aNodeStack.push_front(o3tl::make_unique<SmFontNode>(aToken));
@@ -1822,7 +1825,10 @@ void SmParser::DoFont()
             NextToken();
         }
         else
+        {
             Error(SmParseError::FontExpected);
+            return;
+        }
     } while (m_aCurToken.eType == TFONT);
 
     m_aNodeStack.push_front(o3tl::make_unique<SmFontNode>(aToken));