secp256k1/src/tests.c: Properly handle sscanf return value

This pull request fixes a bug which allows the `sh` variable to be used uninitialized when sscanf returns EOF.

Signed-off-by: Mustapha Abiola <mustapha@trilemma.net>

src/tests.c

@@ -5180,7 +5180,7 @@ int main(int argc, char **argv) {
         const char* ch = argv[2];
         while (pos < 16 && ch[0] != 0 && ch[1] != 0) {
             unsigned short sh;
-            if (sscanf(ch, "%2hx", &sh)) {
+            if ((sscanf(ch, "%2hx", &sh)) == 1) {
                 seed16[pos] = sh;
             } else {
                 break;