sha1_position: convert do-while to while

If we enter the sha1_position() function with "lo == hi",
we have no elements. But the do-while loop means that we'll
enter the loop body once anyway, picking "mi" at that same
value and comparing nonsense to our desired key. This is
unlikely to match in practice, but we still shouldn't be
looking at the memory in the first place.

This bug is inherited from git.git; it was fixed there in
e01580cfe01526ec2c4eb4899f776a82ade7e0e1.

src/sha1_lookup.c

@@ -232,7 +232,7 @@ int sha1_position(const void *table,
 {
 	const unsigned char *base = table;
 
-	do {
+	while (lo < hi) {
 		unsigned mi = (lo + hi) / 2;
 		int cmp = git_oid__hashcmp(base + mi * stride, key);
 
@@ -243,7 +243,7 @@ int sha1_position(const void *table,
 			hi = mi;
 		else
 			lo = mi+1;
-	} while (lo < hi);
+	}
 
 	return -((int)lo)-1;
 }