Use the optval function in the log_message routine

This commit replaces the if-else-block handling the optional prefix
arguments. The colon which was previously appended, if the prefix
argument was present, is only appended if the prefix argument contains
any characters. Previously, prefix='' would end up getting extended to
pref=': '.

Author: Carl Burkert

src/stdlib_logger.f90

@@ -1147,11 +1147,8 @@ subroutine log_message( self, message, module, procedure, prefix )
         character(:), allocatable :: d_and_t, m_and_p, pref
         character(:), allocatable :: buffer
 
-        if ( present(prefix) ) then
-            pref = prefix // ': '
-        else
-            pref = ''
-        end if
+        pref = optval(prefix, '')
+        if ( len(pref) > 0 ) pref = pref // ': '
 
         if ( self % time_stamp ) then
             d_and_t = time_stamp() // ': '