fix #K4088 and #K4089 - ring formatting

+/-N for charged rings with known enchantment was clobbering the
BUC formatting that had occurred earlier.  #K4088 thought it was a
problem with the implicit_cursed option; followup #K4089 from same
user correctly pointed out that the problem was present for any BUC
state.

This is the same line of code that inadvertently omitted the space
between +/-N and "ring of <type>".  That was fixed by commit
1a2b2a8cae a couple of days ago.

While in doname(), fix a potential issue calling corpse_xname().
That assigns a new value to gx.xnamep, clobbering the value that
doname() relied on when it was first called (but doesn't look at
again, so doesn't matter now but could conceivably in the future).

doc/fixes3-7-0.txt

@@ -1845,6 +1845,8 @@ stacked pair of potions of healing in supply chest weighed same as one potion;
 	weight updated as their stack was split; if close to an encumbrance
 	threshold, new weight would trigger an encumbrance increase message,
 	immediately followed a decrease message when the potion got used up
+bounds checking for object name formatting inadvertently introduced a change
+	that hid BUC prefix for charged rings
 
 
 Fixes to 3.7.0-x Platform and/or Interface Problems Exposed Via git Repository