MINOR: chunk: remove impossible tests on negative chunk->data

Since commit 843b7cb ("MEDIUM: chunks: make the chunk struct's fields
match the buffer struct") a chunk length is unsigned so we can remove
negative size checks.
diff --git a/src/chunk.c b/src/chunk.c
index 7e90ee6..303d3fd 100644
--- a/src/chunk.c
+++ b/src/chunk.c
@@ -167,7 +167,7 @@
 	va_list argp;
 	int ret;
 
-	if (chk->data < 0 || !chk->area || !chk->size)
+	if (!chk->area || !chk->size)
 		return 0;
 
 	va_start(argp, fmt);
@@ -193,9 +193,6 @@
 	int olen, free;
 	char c;
 
-	if (dst->data < 0)
-		return dst->data;
-
 	olen = dst->data;
 
 	for (i = 0; i < src->data; i++) {
@@ -238,9 +235,6 @@
 	int olen, free;
 	char c;
 
-	if (dst->data < 0)
-		return dst->data;
-
 	olen = dst->data;
 
 	for (i = 0; i < src->data; i++) {