vasnprintf: fix 2007-11-03 fix

[Bruno Haible]

The vasnprintf fix from 2007-11-03 has the effect that when a reallocation
is needed, the snprintf() result will fit but the test _whether_ the snprintf()
result did fit fails, so that another round of reallocation is performed.
Inefficient...

2007-11-09  Bruno Haible  <address@hidden>

        * lib/vasnprintf.c (VASNPRINTF): Increase reallocation of snprintf
        buffer. Fixes an inefficiency introduced on 2007-11-03.

--- lib/vasnprintf.c.orig       2007-11-09 12:48:01.000000000 +0100
+++ lib/vasnprintf.c    2007-11-09 12:44:50.000000000 +0100
@@ -4294,13 +4294,19 @@
                          goto overflow;
                        else
                          {
-                           /* Need at least count * sizeof (TCHAR_T) bytes.
-                              But allocate proportionally, to avoid looping
+                           /* Need at least (count + 1) * sizeof (TCHAR_T)
+                              bytes.  (The +1 is for the trailing NUL.)
+                              But ask for (count + 2) * sizeof (TCHAR_T)
+                              bytes, so that in the next round, we likely get
+                                maxlen > (unsigned int) count + 1
+                              and so we don't get here again.
+                              And allocate proportionally, to avoid looping
                               eternally if snprintf() reports a too small
                               count.  */
                            size_t n =
                              xmax (xsum (length,
-                                         (count + TCHARS_PER_DCHAR - 1)
+                                         ((unsigned int) count + 2
+                                          + TCHARS_PER_DCHAR - 1)
                                          / TCHARS_PER_DCHAR),
                                    xtimes (allocated, 2));