Shell: Correctly auto format command lines consisting of only whitespace

When the command line consists of only whitespace characters, the auto
formatter basically tries to trim the trailing whitespace. But due to a
subtle bug of decrementing an iterator (which is an unsigned integer)
past 0 (or the start of the buffer), that operation resulted in an index
out of bounds error and a crash.

Userland/Shell/Formatter.h

@@ -30,8 +30,11 @@ public:
             return;
 
         size_t offset = 0;
-        for (auto ptr = m_source.end() - 1; ptr >= m_source.begin() && isspace(*ptr); --ptr)
+        for (auto ptr = m_source.end() - 1; isspace(*ptr); --ptr) {
             ++offset;
+            if (ptr == m_source.begin())
+                break;
+        }
 
         m_trivia = m_source.substring_view(m_source.length() - offset, offset);
     }