[libc] Fix memory leak in MPFRWrapper cospif with MPFR pre 4.2. (llvm#114415)

Author: lntue

libc/utils/MPFRWrapper/MPFRUtils.cpp

@@ -262,6 +262,7 @@ class MPFRNumber {
 
       int d = mpz_tstbit(integer, 0);
       mpfr_set_si(result.value, d ? -1 : 1, mpfr_rounding);
+      mpz_clear(integer);
       return result;
     }