fix st_fuzzed_sig

in case all the bytes of the signature were removed, there are
no bytes to change, so the selection of which bytes to change fails

run that code only if there is anything left of the old signature

Author: tomato42

src/ecdsa/test_malformed_sigs.py

@@ -82,12 +82,13 @@ def st_fuzzed_sig(draw, keys_and_sigs):
     note("Remove bytes: {0}".format(to_remove))
 
     # decide which bytes of the original signature should be changed
-    xors = draw(st.dictionaries(
-        st.integers(min_value=0, max_value=len(sig)-1),
-        st.integers(min_value=1, max_value=255)))
-    for i, val in xors.items():
-        sig[i] ^= val
-    note("xors: {0}".format(xors))
+    if sig:
+        xors = draw(st.dictionaries(
+            st.integers(min_value=0, max_value=len(sig)-1),
+            st.integers(min_value=1, max_value=255)))
+        for i, val in xors.items():
+            sig[i] ^= val
+        note("xors: {0}".format(xors))
 
     # decide where new data should be inserted
     insert_pos = draw(st.integers(min_value=0, max_value=len(sig)))