Fix unsafe type conversion (!74)

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## What is the bug
The way of type conversion [here](https://gitlab.dkrz.de/icon-libraries/libfortran-support/-/blame/55913610ae02141850eb92c2ccc3f58759344646/src/mo_expression.F90?page=2#L1073

) is not suggested.
## How do you fix it
Convert types specifically. 

## How urgent is the bugfix
- [x] I need it as soon as possible
- [ ] I can wait for a couple of days
- [ ] None of my current codes is directly affected

## Mandatory steps before review
- [ ] Gitlab CI passes _(Hint: use `make format` for linting)_ 
- [x] Bugfix is covered by additional unit tests
- [x] Mark the merge request as ready by removing `Draft:`

## Mandatory steps before merge
- [ ] Test coverage does not decrease
- [ ] Reviewed by a maintainer
- [ ] Incorporate review suggestions
- [ ] Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

**You are not supposed to merge this request by yourself, the maintainers of fortan-support take care of this action!**

Co-authored-by: Marek Jacob <1129-b380572@users.noreply.gitlab.dkrz.de>
Approved-by: Marek Jacob <1129-b380572@users.noreply.gitlab.dkrz.de>
Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: bugfix

src/mo_expression.F90

@@ -1053,8 +1053,8 @@ CONTAINS
     INTEGER :: exponent
     CALL result_stack%pop(arg2)
     CALL result_stack%pop(arg1)
-    exponent = arg2
-    IF (arg2 == exponent) THEN
+    exponent = NINT(arg2)
+    IF (arg2 == REAL(exponent, real_kind)) THEN
       arg1 = arg1**exponent
     ELSE
       arg1 = arg1**arg2
@@ -1070,8 +1070,8 @@ CONTAINS
     INTEGER, DIMENSION(:, :), ALLOCATABLE :: exponent
     CALL result_stack%pop(arg2)
     CALL result_stack%pop(arg1)
-    exponent = arg2
-    IF (ALL(arg2 == exponent)) THEN
+    exponent = NINT(arg2)
+    IF (ALL(arg2 == REAL(exponent, real_kind))) THEN
       arg1 = arg1**exponent
     ELSE
       arg1 = arg1**arg2
@@ -1087,8 +1087,8 @@ CONTAINS
     INTEGER, DIMENSION(:, :, :), ALLOCATABLE :: exponent
     CALL result_stack%pop(arg2)
     CALL result_stack%pop(arg1)
-    exponent = arg2
-    IF (ALL(arg2 == exponent)) THEN
+    exponent = NINT(arg2)
+    IF (ALL(arg2 == REAL(exponent, real_kind))) THEN
       arg1 = arg1**exponent
     ELSE
       arg1 = arg1**arg2