fix two issues found by clang's static analyser

scan/scan_asn1derlength.c

@@ -46,7 +46,7 @@ size_t scan_asn1derlengthvalue(const char* src,size_t len,unsigned long long* va
 size_t scan_asn1derlength(const char* src,size_t len,unsigned long long* value) {
   unsigned long long l;
   size_t i=scan_asn1derlengthvalue(src,len,&l);
-  if (l > len-i) return 0;		/* make sure data would fit into buffer */
+  if (!i || l > len-i) return 0;		/* make sure data would fit into buffer */
   *value=l;
   return i;
 }

scan/scan_varint.c

@@ -4,7 +4,6 @@ size_t scan_varint(const char* in,size_t len, unsigned long long* n) {
   size_t i;
   unsigned long long l;
   if (len==0) return 0;
-  l=0;
   for (l=0, i=0; i<len; ++i) {
     l+=(in[i]&0x7f) << (i*7);
     if (!(in[i]&0x80)) {