fix the arguments of inquire routine

The Fortran standard says "The value of a file-unit-number shall
identify a valid unit.".
Once the newunit value generated by open(newunit=..) is not
connected to a unit, it is not valid.
When the unit number is not valid, the compiler can do anything
from return .TRUE., .FALSE., raise an error. Therefore, I added the iostat
argument for the inquire routine.

Author: sakamoti

src/stdlib_logger.f90

@@ -308,6 +308,8 @@ subroutine add_log_unit( self, unit, stat )
         integer :: lun
         character(12) :: specifier
         logical :: question
+        integer :: istat
+        character(len=80) :: msgtxt
 
         call validate_unit()
         if ( present(stat) ) then
@@ -350,7 +352,8 @@ subroutine validate_unit()
             end if
 
 ! Check that unit is opened
-            inquire( unit, opened=question )
+            inquire( unit, opened=question, iostat=istat, iomsg=msgtxt)
+            if(istat/=0) question=.false.
             if ( .not. question ) then
                 if ( present(stat) ) then
                     stat = unopened_in_error

src/tests/logger/test_stdlib_logger.f90

@@ -417,6 +417,8 @@ end subroutine test_adding_log_files
     subroutine test_removing_log_units()
 
         logical :: opened
+        integer :: istat
+        character(len=80) :: msgtxt
 
         print *
         print *, 'running test_removing_log_units'
@@ -462,7 +464,8 @@ subroutine test_removing_log_units()
 
         end if
 
-        inquire( unit4, opened=opened )
+        inquire( unit4, opened=opened ,iostat=istat,iomsg=msgtxt)
+        if(istat/=0) opened=.false.
         if ( opened ) then
             error stop 'UNIT4 is opened contrary to expectations.'