cmd/cgo: allow C.malloc(0) always

Because we can, and because it otherwise might crash
the program if we think we're out of memory.

Fixes #6390.

R=golang-dev, iant, minux.ma
CC=golang-dev
https://golang.org/cl/13345048

misc/cgo/test/cgo_test.go

@@ -46,5 +46,6 @@ func Test3250(t *testing.T)                { test3250(t) }
 func TestCallbackStack(t *testing.T)       { testCallbackStack(t) }
 func TestFpVar(t *testing.T)               { testFpVar(t) }
 func Test4339(t *testing.T)                { test4339(t) }
+func Test6390(t *testing.T)                { test6390(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

src/cmd/cgo/out.go

@@ -1225,6 +1225,8 @@ Slice GoBytes(char *p, int32_t n) {
 extern void runtime_throw(const char *):
 void *Cmalloc(size_t n) {
         void *p = malloc(n);
+        if(p == NULL && n == 0)
+                p = malloc(1);
         if(p == NULL)
                 runtime_throw("runtime: C malloc failed");
         return p;

src/pkg/runtime/cgo/gcc_util.c

@@ -14,6 +14,8 @@ x_cgo_malloc(void *p)
 	} *a = p;
 
 	a->ret = malloc(a->n);
+	if(a->ret == NULL && a->n == 0)
+		a->ret = malloc(1);
 }
 
 /* Stub for calling free from Go */