src/runtime: fix nil map dereference

Operations on nil maps are accepted and shouldn't
panic. The base hashmapGet/hashmapDelete handled
nil-maps correctly, but the hashmapBinary versions
could segfault accessing the nil map while trying
to hash the key.

Fixes #2341

testdata/map.go

@@ -57,6 +57,10 @@ func main() {
 		println(k) // unreachable
 	}
 
+	var nilbinmap map[uint16]int
+	delete(nilbinmap, 4)
+	println("nilbinmap:", nilbinmap[5])
+
 	arrKey := ArrayKey([4]byte{4, 3, 2, 1})
 	println(testMapArrayKey[arrKey])
 	testMapArrayKey[arrKey] = 5555

testdata/map.txt

@@ -53,6 +53,7 @@ true false 0
 nilmap: 0
 4
 42
+nilbinmap: 0
 4321
 5555
 itfMap[3]: 0