fixup! Pass environment variables with empty values

The patch was incomplete. It resulted in settings like `SHELL=` to
*skip* the variable name and the equal sign and instead insert a NUL,
i.e. instead of putting `SHELL=\0` into the environment block, it would
append `\0\0` which would be interpreted as the end of the environment
block instead.

Funnily enough, this was only noticed years later, when Git's `git
rebase -i` progress was changed to clear the line differently depending
on the `TERM` setting, and t/t3404-rebase-interactive.sh triggered the
bug by setting `SHELL` to the empty value, which due to the bug fixed
here would drop `TERM=dumb` by mistake.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/environ.cc

@@ -1323,7 +1323,7 @@ build_env (const char * const *envp, PWCHAR &envblock, int &envc,
 	    continue;
 
 	  /* See if this entry requires posix->win32 conversion. */
-	  conv = getwinenv (*srcp, rest, &temp);
+	  conv = !*rest ? NULL : getwinenv (*srcp, rest, &temp);
 	  if (conv)
 	    {
 	      p = conv->native;	/* Use win32 path */
@@ -1337,7 +1337,7 @@ build_env (const char * const *envp, PWCHAR &envblock, int &envc,
 		}
 	    }
 #ifdef __MSYS__
-	  else if (!keep_posix) {
+	  else if (!keep_posix && *rest) {
 	    char *win_arg = arg_heuristic_with_exclusions
 		   (*srcp, msys2_env_conv_excl_env, msys2_env_conv_excl_count);
 	    debug_printf("WIN32_PATH is %s", win_arg);