patch: segfault fix. Closes 7916

Fix segfault on this case (malformed --- line):

    -- dwarves.orig 2015-02-25 01:45:27.753000000 +0000
    +++ dwarves     2015-02-25 01:46:08.199000000 +0000
    @@ -1,7 +1,7 @@
     Bashful
     Doc
     Dopey
    -Grouchy
    +Grumpy
     Happy
     Sleepy
     Sneezy

function                                             old     new   delta
patch_main                                          1903    1957     +54

Signed-off-by: Denys Vlasenko <[email protected]>

editors/patch.c

@@ -345,6 +345,8 @@ static int apply_one_hunk(void)
 // state 1: Found +++ file indicator, look for @@
 // state 2: In hunk: counting initial context lines
 // state 3: In hunk: getting body
+// Like GNU patch, we don't require a --- line before the +++, and
+// also allow the --- after the +++ line.
 
 int patch_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int patch_main(int argc UNUSED_PARAM, char **argv)
@@ -462,6 +464,14 @@ int patch_main(int argc UNUSED_PARAM, char **argv)
 			TT.context = 0;
 			state = 2;
 
+			// If the --- line is missing or malformed, either oldname
+			// or (for -R) newname could be NULL -- but not both.  Like
+			// GNU patch, proceed based on the +++ line, and avoid SEGVs.
+			if (!oldname)
+				oldname = xstrdup("MISSING_FILENAME");
+			if (!newname)
+				newname = xstrdup("MISSING_FILENAME");
+
 			// If this is the first hunk, open the file.
 			if (TT.filein == -1) {
 				int oldsum, newsum, empty = 0;