InputText: fixed gross buffer underflow introduced by a82f66a. (#3237, #952, #1062, #7363)

Would typically not crash by detected by sanitinizers.

imgui_widgets.cpp

@@ -3992,7 +3992,7 @@ static ImVec2 InputTextCalcTextSize(ImGuiContext* ctx, const char* text_begin, c
 namespace ImStb
 {
 static int     STB_TEXTEDIT_STRINGLEN(const ImGuiInputTextState* obj)                             { return obj->TextLen; }
-static char    STB_TEXTEDIT_GETCHAR(const ImGuiInputTextState* obj, int idx)                      { IM_ASSERT(idx <= obj->TextLen); return obj->TextSrc[idx]; }
+static char    STB_TEXTEDIT_GETCHAR(const ImGuiInputTextState* obj, int idx)                      { IM_ASSERT(idx >= 0 && idx <= obj->TextLen); return obj->TextSrc[idx]; }
 static float   STB_TEXTEDIT_GETWIDTH(ImGuiInputTextState* obj, int line_start_idx, int char_idx)  { unsigned int c; ImTextCharFromUtf8(&c, obj->TextSrc + line_start_idx + char_idx, obj->TextSrc + obj->TextLen); if ((ImWchar)c == '\n') return IMSTB_TEXTEDIT_GETWIDTH_NEWLINE; ImGuiContext& g = *obj->Ctx; return g.FontBaked->GetCharAdvance((ImWchar)c) * g.FontBakedScale; }
 static char    STB_TEXTEDIT_NEWLINE = '\n';
 static void    STB_TEXTEDIT_LAYOUTROW(StbTexteditRow* r, ImGuiInputTextState* obj, int line_start_idx)

imstb_textedit.h

@@ -582,7 +582,7 @@ static void stb_textedit_find_charpos(StbFindState *find, IMSTB_TEXTEDIT_STRING
       STB_TEXTEDIT_LAYOUTROW(&r, str, i);
       if (n < i + r.num_chars)
          break;
-      if (str->LastMoveDirectionLR == ImGuiDir_Right && str->Stb->cursor == i + r.num_chars && STB_TEXTEDIT_GETCHAR(str, i + r.num_chars - 1) != STB_TEXTEDIT_NEWLINE) // [DEAR IMGUI] Wrapping point handling
+      if (str->LastMoveDirectionLR == ImGuiDir_Right && str->Stb->cursor > 0 && str->Stb->cursor == i + r.num_chars && STB_TEXTEDIT_GETCHAR(str, i + r.num_chars - 1) != STB_TEXTEDIT_NEWLINE) // [DEAR IMGUI] Wrapping point handling
          break;
       if (i + r.num_chars == z && z > 0 && STB_TEXTEDIT_GETCHAR(str, z - 1) != STB_TEXTEDIT_NEWLINE)  // [DEAR IMGUI] special handling for last line
          break;   // [DEAR IMGUI]