LibC: Stop stdio from adding null terminators out of bounds (#685)

When using the bounded string operations (e.g. snprintf), the null
terminator was always being written even if there was no space for
it (or indeed any valid buffer at all)

This overwriting caused segmentation faults and memory corruption

Libraries/LibC/stdio.cpp

@@ -395,7 +395,6 @@ int sprintf(char* buffer, const char* fmt, ...)
     va_list ap;
     va_start(ap, fmt);
     int ret = vsprintf(buffer, fmt, ap);
-    buffer[ret] = '\0';
     va_end(ap);
     return ret;
 }
@@ -413,7 +412,9 @@ int vsnprintf(char* buffer, size_t size, const char* fmt, va_list ap)
 {
     __vsnprintf_space_remaining = size;
     int ret = printf_internal(sized_buffer_putch, buffer, fmt, ap);
-    buffer[ret] = '\0';
+    if (__vsnprintf_space_remaining) {
+	    buffer[ret] = '\0';
+    }
     return ret;
 }
 
@@ -422,7 +423,6 @@ int snprintf(char* buffer, size_t size, const char* fmt, ...)
     va_list ap;
     va_start(ap, fmt);
     int ret = vsnprintf(buffer, size, fmt, ap);
-    buffer[ret] = '\0';
     va_end(ap);
     return ret;
 }