FORTIFY_SOURCE: emphasize prevention in error messages.

FORTIFY_SOURCE prevents buffer overflows from occurring.
However, the error message often implies that we only
detect it, not prevent it.

Bring more clarity to the error messages by emphasizing
prevention over detection.

Change-Id: I5f3e1478673bdfc589e6cc4199fce8e52e197a24

libc/bionic/__vsnprintf_chk.cpp

@@ -51,7 +51,7 @@ extern "C" int __vsnprintf_chk(
         va_list va)
 {
     if (__predict_false(supplied_size > dest_len_from_compiler)) {
-        __fortify_chk_fail("vsnprintf buffer overflow", 0);
+        __fortify_chk_fail("vsnprintf buffer overflow prevented", 0);
     }
 
     return vsnprintf(dest, supplied_size, format, va);