Issue #5415: compensate for null byte

When exporting a C source file with runtime length encoding, the
C-string's array size does not accomodate for the null byte. However,
GIMP accomodates for the NULL byte in it's NON-RLE export, suggesting
that this has been a mere oversight for RLE.

This can cause at the worst a compile-time error and at least a warning
from the compiler.

(cherry picked from commit 4d528f297f)

plug-ins/common/file-csource.c

@@ -792,7 +792,7 @@ save_image (GFile   *file,
       if (config->use_rle)
         {
           if (! print (output, error,
-                       "%u] =\n",
+                       "%u + 1] =\n",
                        (guint) (img_buffer_end - img_buffer)))
             goto fail;
         }