libc/stdlib: Error Fix

1.Fix an error in mkstemp() the could result in an infinite loop.
2.Fix for wrong output in some cases.
For Example:
 1) input: "FILEXX"
    output: "FILE00" and repeats same output for further invocations of mkstemp().
            But, the ouput has to be FILE01, FILE02, ...., FILEZZ.
 2)input: "FILEXXXXXX"
   output: "FILE100000", for next invocation "FILE200000" and so on
           But it's good, if the ouput goes like FILE000001, FILE000002, ..., FILE000101, ...

Signed-off-by: Lokesh B V <lokeshbv333@gmail.com>