patch 8.1.0886: compiler warning for NULL pointer and condition always true

Problem:    Compiler warning for adding to NULL pointer and a condition that
            is always true.
Solution:   Check for NULL pointer before adding. Remove useless "if".
            (Friedirch, closes #3913)
diff --git a/src/dosinst.c b/src/dosinst.c
index 5b05598..7025eb6 100644
--- a/src/dosinst.c
+++ b/src/dosinst.c
@@ -291,16 +291,17 @@
 {
     char	*bp = *destination;
     size_t	indir_l = strlen(installdir);
-    char	*cp = bp + indir_l;
+    char	*cp;
     char	*tmpname;
     char	*farname;
 
     /*
      * No action needed if exe not found or not in this directory.
      */
-    if (bp == NULL
-	    || strnicmp(bp, installdir, indir_l) != 0
-	    || strchr("/\\", *cp++) == NULL
+    if (bp == NULL || strnicmp(bp, installdir, indir_l) != 0)
+	return;
+    cp = bp + indir_l;
+    if (strchr("/\\", *cp++) == NULL
 	    || strchr(cp, '\\') != NULL
 	    || strchr(cp, '/') != NULL)
 	return;