Fix regression on ARM after Target FP patches

Fix regression on ARM after Target FP patches

Commit edd079d9f6ca2f9ad21322b742269aec5de61190 exposed a pre-existing bug
in convert_doublest_to_floatformat.  In the specific case of converting
a zero value to a floatformat using a "special" byteorder (i.e. neither
floatformat_little nor floatformat_big), the output buffer was actually
left uninitialized.

gdb/ChangeLog:
2017-11-03  Ulrich Weigand  <uweigand@de.ibm.com>

	* doublest.c (convert_doublest_to_floatformat): Fix uninitialized
	output when converting a zero value to a special byteorder format.