do not allow malloc(0)

vlib/builtin/builtin.v

@@ -105,7 +105,7 @@ __global nr_mallocs int = 0
 
 [unsafe_fn]
 pub fn malloc(n int) byteptr {
-	if n < 0 {
+	if n <= 0 {
 		panic('malloc(<0)')
 	}
 	$if prealloc {

vlib/builtin/string.v

@@ -1126,6 +1126,9 @@ pub fn (s []string) join_lines() string {
 
 // reverse will return a new reversed string.
 pub fn (s string) reverse() string {
+	if s.len == 0 {
+		return ''
+	}
 	mut res := string {
 		len: s.len
 		str: malloc(s.len)

vlib/encoding/base64/base64.v

@@ -24,7 +24,11 @@ const (
  * NB: if you need to decode many strings repeatedly, take a look at decode_in_buffer too.
  */
 pub fn decode(data string) string {
-	buffer := malloc( data.len * 3 / 4 )
+	size := data.len * 3 / 4
+	if size <= 0 {
+		return ''
+	}
+	buffer := malloc(size)
 	return tos(buffer, decode_in_buffer(data, buffer) )
 }
 
@@ -36,7 +40,11 @@ pub fn decode(data string) string {
  * NB: if you need to encode many strings repeatedly, take a look at encode_in_buffer too.
  */
 pub fn encode(data string) string {
-	buffer := malloc( 4 * ((data.len + 2) / 3) )
+	size := 4 * ((data.len + 2) / 3)
+	if size <= 0 {
+		return ''
+	}
+	buffer := malloc(size)
 	return tos(buffer, encode_in_buffer(data, buffer))
 }