Rename 'free' variable to 'nfree' to not clash with free()

Rename 'free' variable to 'nfree' to not clash with free()

The beauty and horrors of the C - the compiler properly detects variable
shadowing, but you can freely shadow a standard function 'free()' with
variable called 'free'.  And if you reference 'free()' just as 'free'
you get the function pointer which means you can do also pointer
arithmetics, so 'free > 0' is always valid even when you delete the
local variable.

Replace the local variables 'free' with a name that doesn't shadow the
'free()' function to prevent future hard to detect bugs.