fix: index handling in chomp (fortran-lang#1012)

* index handling

* Update src/stdlib_strings.fypp

* add a simple test case

* remove short-circuiting logic

---------

Co-authored-by: Federico Perini <federico.perini@gmail.com>

Author: wassup05

src/stdlib_strings.fypp

@@ -302,12 +302,21 @@ contains
 
         last = len(string)
         nsub = len(substring)
-        if (nsub > 0) then
-            do while(string(last-nsub+1:last) == substring)
-                last = last - nsub
+        if (nsub > 0 .and. nsub <= last) then
+            do while(last >= nsub)
+                if (string(last-nsub+1:last) == substring) then
+                    last = last - nsub
+                else
+                    exit
+                end if
             end do
         end if
-        chomped_string = string(1:last)
+
+        if (last <= 0) then
+            chomped_string = ''
+        else
+            chomped_string = string(1:last)
+        end if        
 
     end function chomp_substring_char_char
 

test/string/test_string_strip_chomp.f90

@@ -151,6 +151,8 @@ subroutine test_chomp_substring_char(error)
         call check(error, chomp("hellooooo", "oo") == "hello")
         if (allocated(error)) return
         call check(error, chomp("hellooooo", substring="oo") == "hello")
+        if (allocated(error)) return
+        call check(error, chomp("helhel", substring="hel") == "")
     end subroutine test_chomp_substring_char
 
     subroutine test_chomp_substring_string(error)