Stop memory leak in error paths in XawTextSinkConvertPropertyList()

Fixes issues reported by Oracle Parfait static analyzer:

Error: Memory leak
   Memory leak [memory-leak] (CWE 401):
      Memory leak of pointer pointer allocated with XtCalloc(1, 144)
        at line 1586 of lib/libXaw/src/TextSink.c in function 'XawTextSinkConvertPropertyList'.
          pointer allocated at line 1570 with XtCalloc(1, 144)
          pointer leaks when XawFindArgVal(params, "font") != NULL at line 1573
              and argval->value != NULL at line 1574
              and XLoadQueryFont(screen->display, argval->value) == NULL at line 1577.
   Memory leak [memory-leak] (CWE 401):
      Memory leak of pointer pointer allocated with XtCalloc(1, 144)
        at line 1605 of lib/libXaw/src/TextSink.c in function 'XawTextSinkConvertPropertyList'.
          pointer allocated at line 1570 with XtCalloc(1, 144)
          pointer leaks when argval->value == NULL at line 1574
              and XawFindArgVal(params, "foreground") != NULL at line 1593
              and argval->value != NULL at line 1594
              and XAllocNamedColor(...) == 0 at line 1595.
   Memory leak [memory-leak] (CWE 401):
      Memory leak of pointer pointer allocated with XtCalloc(1, 144)
        at line 1622 of lib/libXaw/src/TextSink.c in function 'XawTextSinkConvertPropertyList'.
          pointer allocated at line 1570 with XtCalloc(1, 144)
          pointer leaks when argval->value == NULL at line 1574
              and XawFindArgVal(params, "background") != NULL at line 1610
              and argval->value != NULL at line 1611
              and XAllocNamedColor(...) == 0 at line 1612.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
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