Fwd: UBSAN warnings for u16-to-u8 and u8-to-u16

[Dmitri A. Sergatskov]

When Octave (which uses gnulib) is compiled with UBSAN I see the following
while running its test suite:

../../libgnu/unistr/u8-to-u16.c:80:34: runtime error: applying zero offset
to null pointer SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
../../libgnu/unistr/u8-to-u16.c:80:34
../../libgnu/unistr/u16-to-u8.c:80:33: runtime error: applying zero offset
to null pointer SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
../../libgnu/unistr/u16-to-u8.c:80:33

Looking at the sourse I see the line
count = u16_uctomb(result + length, uc, allocated - length);
So at the begging of the loop "length" is 0 but "result" is NULL, so
technically this is NULL pointer arithmetics, and UBSAN does not like it.

I am not even 100% sure this is a bug, but the diffs:

@@ -77,7 +77,11 @@
       s += count;

       /* Store it in the output string.  */
-      count = u16_uctomb (result + length, uc, allocated - length);
+      if (result != NULL)
+        count = u16_uctomb (result + length, uc, allocated - length);
+      else
+        count = -2; /* Need for allocation */
+
       if (count == -1)
         {
           if (!(result == resultbuf || result == NULL))

and

@@ -77,7 +77,11 @@
       s += count;

       /* Store it in the output string.  */
-      count = u8_uctomb (result + length, uc, allocated - length);
+      if (result != NULL)
+        count = u8_uctomb (result + length, uc, allocated - length);
+      else
+        count = -2; /* Need for allocation */
+
       if (count == -1)
         {
           if (!(result == resultbuf || result == NULL))

silenced UBSAN for me.

Dmitri.
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