mg: remove some more warnings

[Brian Callahan]

Hi tech --

A very recent clang says:

/home/brian/llvm/build-build/bin/clang -O2 -pipe  -Wall -DFKEYS -DREGEX -DXKEYS 
-Werror-implicit-function-declaration  -c /usr/src/usr.bin/mg/dired.c
/usr/src/usr.bin/mg/dired.c:216:13: warning: address of array 'curbp->b_fname' 
will always evaluate to 'true' [-Wpointer-bool-conversion]
        if (curbp->b_fname && curbp->b_fname[0] != '\0') {
            ~~~~~~~^~~~~~~ ~~
/usr/src/usr.bin/mg/dired.c:246:13: warning: address of array 'curbp->b_fname' 
will always evaluate to 'true' [-Wpointer-bool-conversion]
        if (curbp->b_fname && curbp->b_fname[0] != '\0') {
            ~~~~~~~^~~~~~~ ~~
2 warnings generated.

Diff below to kill the warnings.

OK?

~Brian

Index: dired.c
===================================================================
RCS file: /cvs/src/usr.bin/mg/dired.c,v
retrieving revision 1.68
diff -u -p -r1.68 dired.c
--- dired.c     16 Nov 2014 04:16:41 -0000      1.68
+++ dired.c     27 Dec 2014 22:07:52 -0000
@@ -213,7 +213,7 @@ dired(int f, int n)
        char             dname[NFILEN], *bufp, *slash;
        struct buffer   *bp;
 
-       if (curbp->b_fname && curbp->b_fname[0] != '\0') {
+       if (curbp->b_fname[0] != '\0') {
                (void)strlcpy(dname, curbp->b_fname, sizeof(dname));
                if ((slash = strrchr(dname, '/')) != NULL) {
                        *(slash + 1) = '\0';
@@ -243,7 +243,7 @@ d_otherwindow(int f, int n)
        struct buffer   *bp;
        struct mgwin    *wp;
 
-       if (curbp->b_fname && curbp->b_fname[0] != '\0') {
+       if (curbp->b_fname[0] != '\0') {
                (void)strlcpy(dname, curbp->b_fname, sizeof(dname));
                if ((slash = strrchr(dname, '/')) != NULL) {
                        *(slash + 1) = '\0';