memo.c might not always be initialized

memo.float_value might change inside of hash_sum.  In case it
flipped from false to true there, and the calculated sum is Inf,
memo.c might not be initialized at all.  This is bad.

Found using memory sanitizer:

==55293==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x55dfb8d6c529 in rb_float_new_inline internal.h:1814:53
    ruby#1 0x55dfb8d1b30c in enum_sum enum.c:4017:18
    ruby#2 0x55dfb86d75ad in call_cfunc_m1 vm_insnhelper.c:2041:12
    ruby#3 0x55dfb864b141 in vm_call_cfunc_with_frame vm_insnhelper.c:2207:11
    ruby#4 0x55dfb85e843d in vm_call_cfunc vm_insnhelper.c:2225:12
    ruby#5 0x55dfb85e08f3 in vm_call_method_each_type vm_insnhelper.c:2560:9
    ruby#6 0x55dfb85de9c7 in vm_call_method vm_insnhelper.c:2686:13
    ruby#7 0x55dfb849eac6 in vm_call_general vm_insnhelper.c:2730:12
    ruby#8 0x55dfb8686103 in vm_sendish vm_insnhelper.c:3623:11
    ruby#9 0x55dfb84dc29e in vm_exec_core insns.def:789:11

enum.c

@@ -3997,6 +3997,10 @@ enum_sum(int argc, VALUE* argv, VALUE obj)
         memo.f = RFLOAT_VALUE(memo.v);
         memo.c = 0.0;
     }
+    else {
+        memo.f = 0.0;
+        memo.c = 0.0;
+    }
 
     if (RTEST(rb_range_values(obj, &beg, &end, &excl))) {
         if (!memo.block_given && !memo.float_value &&