34543: Prevent crash on garbage bytes inside $(...)

Garbage input (nul bytes, etc.) can cause the $(...) parser to become
confused during look-ahead and attempt to back up the input too far.
This commit catches the error but does not fix the underlying cause.

ChangeLog

@@ -1,3 +1,8 @@
+2015-02-14  Barton E. Schaefer  <schaefer@zsh.org>
+
+	* 34543: Src/input.c, Src/lex.c: Fix crash on garbage bytes
+	inside $(...)
+
 2015-02-14  Mikael Magnusson  <mikachu@gmail.com>
 
 	* unposted: Doc/Zsh/prompt.yo: Fix typo from 28487.

Src/input.c

@@ -393,12 +393,14 @@ inungetc(int c)
 	    if (((inbufflags & INP_LINENO) || !strin) && c == '\n')
 		lineno--;
 	}
-#ifdef DEBUG
         else if (!(inbufflags & INP_CONT)) {
+#ifdef DEBUG
 	    /* Just for debugging */
 	    fprintf(stderr, "Attempt to inungetc() at start of input.\n");
-	}
 #endif
+	    zerr("Garbled input at %c (binary file as commands?)", c);
+	    return;
+	}
 	else {
 	    /*
 	     * The character is being backed up from a previous input stack

Src/lex.c

@@ -503,13 +503,15 @@ cmd_or_math(int cs_type)
     /* else unsuccessful: unget the whole thing */
     hungetc(c);
     lexstop = 0;
-    while (lexbuf.len > oldlen) {
+    while (lexbuf.len > oldlen && !errflag) {
 	lexbuf.len--;
 	hungetc(itok(*--lexbuf.ptr) ?
 		ztokens[*lexbuf.ptr - Pound] : *lexbuf.ptr);
     }
+    if (errflag)
+	return 2;
     hungetc('(');
-    return 0;
+    return errflag ? 2 : 0;
 }