Fix the termination of the readlink result

nread is the number of bytes put into the buffer, let's terminate it there
instead of one byte over. This only worked because execdir was initialized to
zero.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 34fe17f723)

src/builddir.h

@@ -50,7 +50,7 @@ builddir_lookup(void)
 
 	/* readlink doesn't terminate the string and readlink says
 	   anything past sz is undefined */
-	execdir[++nread] = '\0';
+	execdir[nread] = '\0';
 
 	pathsep = strrchr(execdir, '/');
 	if (!pathsep)