graphics/gimp-app: fix build with clang 15

During an exp-run for llvm 15 (see bug 265425), it turned out that
graphics/gimp-app failed to build with clang 15:

  file-dicom.c:605:26: error: incompatible pointer to integer conversion returning 'void *' from a function with result type 'gint32' (aka 'int') [-Wint-conversion]
                    return NULL;
                           ^~~~

This is because the return statement is a function that should return an
integer, not a pointer. Replace it with the error value -1, as used
elsewhere in the function.

PR:		268790
Approved by:	fluffy (maintainer)
MFH:		2023Q1

graphics/gimp-app/files/patch-plug-ins_common_file-dicom.c

@@ -0,0 +1,11 @@
+--- plug-ins/common/file-dicom.c.orig	2022-06-12 20:26:00 UTC
++++ plug-ins/common/file-dicom.c
+@@ -602,7 +602,7 @@ load_image (const gchar  *filename,
+                                (gchar *) value, samples_per_pixel);
+                   g_free (dicominfo);
+                   fclose (DICOM);
+-                  return NULL;
++                  return -1;
+                 }
+ 
+               break;