vi mode: Disable = and tab at the start of the line (#521)

In vi mode, due to a buffer overflow, <ESC>= as the first input
either produces an invalid completion or crashes the shell. The
easiest fix is to disable <ESC>= as well as <TAB> completion at
the start of the command line, as is already done in emacs mode.

src/cmd/ksh93/edit/vi.c:
- getline(): If cur_virt <= 0 in case '\t', beep and refuse tab
  completion.
- textmod(): Move an 'if' statement checking for INVALID cur_virt
  to include = mode along with * and \ mode.

Co-authored-by: Martijn Dekker <martijn@inlv.org>
Resolves: https://github.com/ksh93/ksh/issues/520

NEWS

@@ -2,6 +2,12 @@ This documents significant changes in the 1.0 branch of ksh 93u+m.
 For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
 
+2022-08-24:
+
+- Fixed a bug that caused ksh in the vi editor mode to crash or produce
+  invalid completions if ESC = was used at the beginning of a line. Both
+  tab and = completions are now disabled at the start of the command line.
+
 2022-08-20:
 
 - Fixed a bug in command line options processing that caused short-form

src/cmd/ksh93/edit/vi.c

@@ -1560,6 +1560,11 @@ static void getline(register Vi_t* vp,register int mode)
 				}
 				vp->ed->e_tabcount = 0;
 			}
+			if(cur_virt <= 0)
+			{
+				ed_ringbell();
+				break;
+			}
 			/* FALLTHROUGH */
 		default:
 		fallback:
@@ -1581,7 +1586,6 @@ static void getline(register Vi_t* vp,register int mode)
 			break;
 		}
 		refresh(vp,INPUT);
-
 	}
 }
 
@@ -2540,10 +2544,10 @@ addin:
 		/* FALLTHROUGH */
 	case '*':		/** do file name expansion in place **/
 	case '\\':		/** do file name completion in place **/
+	case '=':		/** list file name expansions **/
 		if( cur_virt == INVALID )
 			return(BAD);
 		/* FALLTHROUGH */
-	case '=':		/** list file name expansions **/
 		save_v(vp);
 		i = last_virt;
 		++last_virt;

src/cmd/ksh93/include/version.h

@@ -18,7 +18,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.3-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2022-08-20"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2022-08-24"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh.1

@@ -5581,10 +5581,10 @@ or
 character.
 .TP 10
 .BI ^I  " tab"
-Attempts command or file name completion as described above
-and returns to input mode.
-If a partial completion occurs, repeating this will
-behave as if
+Except at the start of the line, attempts command or
+file name completion as described above and returns to
+input mode. If a partial completion occurs, repeating
+this will behave as if
 .B =
 were entered from control mode.
 If no match is found or entered after