Fix UB in ZstdDecompressor.unused_data

Author: emmatyping

Modules/_zstd/decompressor.c

@@ -588,7 +588,9 @@ _zstd_ZstdDecompressor_unused_data_get_impl(ZstdDecompressor *self)
 
     PyMutex_Lock(&self->lock);
 
-    if (!self->eof) {
+    // If we aren't done decompressing or no input data has been given,
+    // return empty bytes
+    if (!self->eof || self->input_buffer == NULL) {
         PyMutex_Unlock(&self->lock);
         return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
     }

Tools/ubsan/suppressions.txt

@@ -18,9 +18,6 @@ shift-base:Modules/_ctypes/cfield.c
 # Modules/_ctypes/cfield.c:640:1: runtime error: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
 signed-integer-overflow:Modules/_ctypes/cfield.c
 
-# Modules/_zstd/decompressor.c:598:56: runtime error: applying non-zero offset 18446744073709551615 to null pointer
-pointer-overflow:Modules/_zstd/decompressor.c
-
 # Modules/_io/stringio.c:350:24: runtime error: addition of unsigned offset to 0x7fd01ec25850 overflowed to 0x7fd01ec2584c
 pointer-overflow:Modules/_io/stringio.c