fix(crypto): Fix out of bounds read in ecdsa_sig_to_der().

[no changelog]

crypto/ecdsa.c

@@ -1159,7 +1159,7 @@ int ecdsa_sig_to_der(const uint8_t *sig, uint8_t *der) {
 
   // process R
   i = 0;
-  while (sig[i] == 0 && i < 32) {
+  while (i < 31 && sig[i] == 0) {
     i++;
   }                      // skip leading zeroes
   if (sig[i] >= 0x80) {  // put zero in output if MSB set
@@ -1182,7 +1182,7 @@ int ecdsa_sig_to_der(const uint8_t *sig, uint8_t *der) {
 
   // process S
   i = 32;
-  while (sig[i] == 0 && i < 64) {
+  while (i < 63 && sig[i] == 0) {
     i++;
   }                      // skip leading zeroes
   if (sig[i] >= 0x80) {  // put zero in output if MSB set

crypto/tests/test_check.c

@@ -6255,6 +6255,11 @@ START_TEST(test_ecdsa_der) {
           "00000000000000000000000000000000000000000000000000000000000000ff",
           "3008020200ee020200ff",
       },
+      {
+          "0000000000000000000000000000000000000000000000000000000000000000",
+          "0000000000000000000000000000000000000000000000000000000000000000",
+          "3006020100020100",
+      },
   };
 
   uint8_t sig[64];