Fix use after free (realloc) in ParseText.cpp

Static analysis detected a use after free (realloc).  The
member variable should use the new string address, and it
is assigned the new string address after the usage.  This
looks to be a copy/paste error sometime in the past.  In
addition, follow the stated convention and null terminate
the string, just in case.
(cherry picked from commit df4aef6)

Signed-off-by: Stuart Morgan <smorgan@mythtv.org>

Fixes #11556

mythtv/libs/libmythfreemheg/ParseText.cpp

@@ -800,12 +800,13 @@ void MHParseText::NextSym()
                             Error("Insufficient memory");
                         }
 
+                        m_String = str;
                         m_String[0] = colourTable[i].r;
                         m_String[1] = colourTable[i].g;
                         m_String[2] = colourTable[i].b;
                         m_String[3] = colourTable[i].t;
-                        m_String = str;
                         m_nStringLength = 4;
+                        m_String[m_nStringLength] = 0;
                         return;
                     }
                 }