runtime: fix _cgo_allocate(0)

Fixes a SWIG bug reported off-list.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/155990043

misc/cgo/test/callback_c_gc.c

@@ -40,6 +40,16 @@ callCgoAllocate(void)
 	struct { size_t n; void *ret; } a;
 	List *l, *head, **tail;
 
+	// Make sure this doesn't crash.
+	// And make sure it returns non-nil.
+	a.n = 0;
+	a.ret = 0;
+	crosscall2(_cgo_allocate, &a, sizeof a);
+	if(a.ret == 0) {
+		fprintf(stderr, "callCgoAllocate: alloc 0 returned nil\n");
+		exit(2);
+	}
+	
 	head = 0;
 	tail = &head;
 	for(i=0; i<100; i++) {

misc/cgo/test/callback_c_gccgo.c

@@ -35,6 +35,13 @@ callCgoAllocate(void)
 	int i;
 	List *l, *head, **tail;
 	
+	// Make sure this doesn't crash.
+	// And make sure it returns non-nil.
+	if(_cgo_allocate(0) == 0) {
+		fprintf(stderr, "callCgoAllocate: alloc 0 returned nil\n");
+		exit(2);
+	}
+
 	head = 0;
 	tail = &head;
 	for(i=0; i<100; i++) {

src/runtime/cgocallback.go

@@ -21,6 +21,9 @@ import "unsafe"
 // Either we need to add types or we need to stop using it.
 
 func _cgo_allocate_internal(len uintptr) unsafe.Pointer {
+	if len == 0 {
+		len = 1
+	}
 	ret := unsafe.Pointer(&make([]unsafe.Pointer, (len+ptrSize-1)/ptrSize)[0])
 	c := new(cgomal)
 	c.alloc = ret