sign-digest: Fixing overflow check and output wording when start is n…

…egative. (Feedback from Bu11etmagnet.)

src/sign-digest.cpp

@@ -30,13 +30,9 @@ sealfield *	RangeErrorCheck	(sealfield *Rec, uint64_t sum[2], mmapfile *Mmap)
 {
   // Idiot-check the range
   if (sum[0]==sum[1]) { return(Rec); } // sure, permit an empty range
-  if (sum[0] < 0)
+  if (sum[0] > Mmap->memsize) // sum went negative
 	{
-	Rec = SealSetText(Rec,"@error","Invalid range; start of range is negative");
-	}
-  if (sum[1] < 0)
-        {
-	Rec = SealSetText(Rec,"@error","Invalid range; end of range is negative");
+	Rec = SealSetText(Rec,"@error","Invalid range; start of range is beyond end of file");
 	}
   if (sum[1] > Mmap->memsize)
         {
@@ -207,12 +203,13 @@ sealfield *	SealDigest	(sealfield *Rec, mmapfile *Mmap)
       else { sum[1] += acc*Addsym; }
 
       // Check the range
-      if ((sum[1] < sum[0]) || (sum[0] < 0) || (sum[1] > Mmap->memsize))
+      // sum is size_t/unsigned. If it goes negative, it will be larger than memsize.
+      if ((sum[1] < sum[0]) || (sum[0] > Mmap->memsize) || (sum[1] > Mmap->memsize))
 	{
 	Rec = SealSetText(Rec,"@error","Invalid range in b='");
 	Rec = SealAddText(Rec,"@error",b);
 	Rec = SealAddText(Rec,"@error","'");
-	if (sum[0] < 0) { Rec = SealAddText(Rec,"@error","; underflow"); }
+	if (sum[0] > Mmap->memsize) { Rec = SealAddText(Rec,"@error","; underflow"); }
 	if (sum[1] > Mmap->memsize) { Rec = SealAddText(Rec,"@error","; overflow"); }
 	if (sum[1] < sum[0]) { Rec = SealAddText(Rec,"@error","; range begins after it ends"); }
 	goto Abort;