Float values divided by zero should honor signs; fix #3766

It also fixes unexpected resurrection of #3745 by #3752

src/numeric.c

@@ -187,12 +187,9 @@ flodivmod(mrb_state *mrb, mrb_float x, mrb_float y, mrb_float *divp, mrb_float *
   mrb_float mod;
 
   if (y == 0.0) {
-    if (x == 0.0) {
-      div = NAN;
-    }
-    else {
-      div = INFINITY;
-    }
+    if (x == 0.0) div = NAN;
+    else if (x > 0.0) div = INFINITY;
+    else if (x < 0.0) div = -INFINITY;
     mod = NAN;
   }
   else {

src/vm.c

@@ -2389,8 +2389,10 @@ RETRY_TRY_BLOCK:
           mrb_int x = mrb_fixnum(regs[a]);
           mrb_int y = mrb_fixnum(regs[a+1]);
           double f;
-          if (y == 0 && x != 0) {
-            f = INFINITY;
+          if (y == 0) {
+            if (x == 0) f = NAN;
+            else if (x > 0) f = INFINITY;
+            else if (x < 0) f = -INFINITY;
           }
           else {
             f = (mrb_float)x / (mrb_float)y;