fix #H4336 - extra space in potions boil message

fix #H4336 - extra space in potions boil message

'Your <single-potion> boils and explodes.'
'One of your <stack-of-potions> boils and explodes.'
'Some of your <stack-of-potions> boil and explode.'
'All of your  <stack-of-potions> boil and explode.'
The last variation had an extra space in the message prefix....

In addition to removing the excess space, this adds
'Both of your <stack-of-potions> boil and explode.'
to be used for the (All == 2) case.

The bug and fix also apply to stacks of potions 'freezing and
shattering' and to stacks of scrolls 'catching fire and burning'.