malloc: Simplify implementation of __malloc_assert

It is prudent not to run too much code after detecting heap
corruption, and __fxprintf is really complex.  The line number
and file name do not carry much information, so it is not included
in the error message.  (__libc_message only supports %s formatting.)
The function name and assertion should provide some context.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>