liblzma: Avoid memcpy(NULL, foo, 0) because it is undefined behavior.

I should have always known this but I didn't. Here is an example
as a reminder to myself:

    int mycopy(void *dest, void *src, size_t n)
    {
        memcpy(dest, src, n);
        return dest == NULL;
    }

In the example, a compiler may assume that dest != NULL because
passing NULL to memcpy() would be undefined behavior. Testing
with GCC 8.2.1, mycopy(NULL, NULL, 0) returns 1 with -O0 and -O1.
With -O2 the return value is 0 because the compiler infers that
dest cannot be NULL because it was already used with memcpy()
and thus the test for NULL gets optimized out.

In liblzma, if a null-pointer was passed to memcpy(), there were
no checks for NULL *after* the memcpy() call, so I cautiously
suspect that it shouldn't have caused bad behavior in practice,
but it's hard to be sure, and the problematic cases had to be
fixed anyway.

Thanks to Jeffrey Walton.

src/liblzma/common/common.c

@@ -99,6 +99,10 @@ lzma_bufcpy(const uint8_t *restrict in, size_t *restrict in_pos,
 	const size_t out_avail = out_size - *out_pos;
 	const size_t copy_size = my_min(in_avail, out_avail);
 
+	// Call memcpy() only if there is something to copy. If there is
+	// nothing to copy, in or out might be NULL and then the memcpy()
+	// call would trigger undefined behavior.
+	if (copy_size > 0)
 		memcpy(out + *out_pos, in + *in_pos, copy_size);
 
 	*in_pos += copy_size;

src/liblzma/lz/lz_decoder.c

@@ -91,11 +91,17 @@ decode_buffer(lzma_coder *coder,
 				in, in_pos, in_size);
 
 		// Copy the decoded data from the dictionary to the out[]
-		// buffer.
+		// buffer. Do it conditionally because out can be NULL
+		// (in which case copy_size is always 0). Calling memcpy()
+		// with a null-pointer is undefined even if the third
+		// argument is 0.
 		const size_t copy_size = coder->dict.pos - dict_start;
 		assert(copy_size <= out_size - *out_pos);
+
+		if (copy_size > 0)
 			memcpy(out + *out_pos, coder->dict.buf + dict_start,
 					copy_size);
+
 		*out_pos += copy_size;
 
 		// Reset the dictionary if so requested by coder->lz.code().

src/liblzma/simple/simple_coder.c

@@ -118,7 +118,15 @@ simple_code(void *coder_ptr, const lzma_allocator *allocator,
 		// coder->pos and coder->size yet. This way the coder can be
 		// restarted if the next filter in the chain returns e.g.
 		// LZMA_MEM_ERROR.
-		memcpy(out + *out_pos, coder->buffer + coder->pos, buf_avail);
+		//
+		// Do the memcpy() conditionally because out can be NULL
+		// (in which case buf_avail is always 0). Calling memcpy()
+		// with a null-pointer is undefined even if the third
+		// argument is 0.
+		if (buf_avail > 0)
+			memcpy(out + *out_pos, coder->buffer + coder->pos,
+					buf_avail);
+
 		*out_pos += buf_avail;
 
 		// Copy/Encode/Decode more data to out[].