win32: fix memory leak

string is allocated by g_malloc, will not be used after putenv, should be
free before return.

Paolo Bonzini <pbonzini@redhat.com> confirmed this is safe under Wine:

"1) the underlying Win32 APIs require separate arguments for the
variable and value; 2) even though in the end Wine stores the
environment as name=value
(http://source.winehq.org/source/dlls/ntdll/env.c), it does so in a
single consecutive block of memory, not as a char* array like POSIX
does.  While (2) might apply only to Wine, (1) surely applies to Windows
as well."
Tested-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NLi Zhi Hui <zhihuili@linux.vnet.ibm.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

os-win32.c

@@ -44,6 +44,13 @@ int setenv(const char *name, const char *value, int overwrite)
         char *string = g_malloc(length);
         snprintf(string, length, "%s=%s", name, value);
         result = putenv(string);
+
+        /* Windows takes a copy and does not continue to use our string.
+         * Therefore it can be safely freed on this platform.  POSIX code
+         * typically has to leak the string because according to the spec it
+         * becomes part of the environment.
+         */
+        g_free(string);
     }
     return result;
 }