mprintf: avoid unsigned integer overflow warning

The overflow has no real world impact.
Just avoid it for "best practice".

Code change suggested by "The Infinnovation Team" and Daniel Stenberg.
Closes #3184

lib/mprintf.c

@@ -835,7 +835,7 @@ static int dprintf_formatf(
           while(width-- > 0)
             OUTCHAR(' ');
 
-        while((len-- > 0) && *str)
+        for(; len && *str; len--)
           OUTCHAR(*str++);
         if(p->flags&FLAGS_LEFT)
           while(width-- > 0)