gh-39135: Fix ginac's ratlog method so log(int(0),2) does not hang
    
Fixes #37794.

It is explained in #37794 that calculating the logarithm of `int(0)`
(zero represented as a python integer) can enter an infinite loop, due
to a bug in ginac's `numeric::ratlog` method. This PR adds an `if`
statement to return negative infinity for this value. (At this point in
the code, the base of the logarithm is known to be a positive integer
that is greater than 1, so the log of 0 is negative infinity, not
positive infinity.)

The PR also adds `log(int(0), 2)` as a doctest and, for good measure,
also `log(int(0), 1/2)`.



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- [x] I have linked a relevant issue or discussion.
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preview.

### ⌛ Dependencies

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URL: #39135
Reported by: DaveWitteMorris
Reviewer(s): Lorenz Panny

Author: Release Manager

src/sage/misc/functional.py

@@ -1137,6 +1137,13 @@ def log(*args, **kwds):
 
         sage: log(0, 2)
         -Infinity
+
+    Check if :issue:`37794` is fixed::
+
+        sage: log(int(0), 2)
+        -Infinity
+        sage: log(int(0), 1/2)
+        +Infinity
     """
     base = kwds.pop('base', None)
     if base:

src/sage/symbolic/ginac/numeric.cpp

@@ -3665,7 +3665,7 @@ const numeric numeric::log(const numeric &b, PyObject* parent) const {
 }
 
 // General log
-// Handle special cases here that return MPZ/MPQ
+// Handle special cases here that return MPZ/MPQ (or an infinity)
 const numeric numeric::ratlog(const numeric &b, bool& israt) const {
         israt = true;
         if (b.is_one()) {
@@ -3688,6 +3688,9 @@ const numeric numeric::ratlog(const numeric &b, bool& israt) const {
                         israt = false;
                         return *_num0_p;
                 }
+                if (v._long == 0) {
+                        return py_funcs.py_eval_neg_infinity();
+                }
                 int c = 0;
                 std::ldiv_t ld;
                 ld.quot = v._long;