gh-120155: Add assertion to sre.c match_getindex() (#120402)

Add an assertion to help static analyzers to detect that i*2 cannot
overflow.

Modules/_sre/sre.c

@@ -2217,6 +2217,8 @@ match_getindex(MatchObject* self, PyObject* index)
         return -1;
     }
 
+    // Check that i*2 cannot overflow to make static analyzers happy
+    assert(i <= SRE_MAXGROUPS);
     return i;
 }