remove short-circuiting logic

Author: wassup05

src/stdlib_strings.fypp

@@ -303,8 +303,12 @@ contains
         last = len(string)
         nsub = len(substring)
         if (nsub > 0 .and. nsub <= last) then
-            do while(last >= nsub .and. string(last-nsub+1:last) == substring)
-                last = last - nsub
+            do while(last >= nsub)
+                if (string(last-nsub+1:last) == substring) then
+                    last = last - nsub
+                else
+                    exit
+                end if
             end do
         end if