Fix incorrect cast in nano malloc

As described in nano-mallocr.c, chunks of heap are represented in memory
as a size (of type long), followed by some optional padding containing a
negative offset to size, followed by the data area.

get_chunk_from_ptr is responsible for taking a pointer to the data area
(as returned by malloc) and finding the start of the chunk. It does this
by assuming there is no padding and trying to read the size, if the size
is negative then it uses that as an offset to find the true size.
Crucially, it reads the padding area as a long.

nano_malloc is responsible for populating the optional padding area. It
does so by casting a pointer to an (int *) and writing the negative
offset into it.

This means that padding is being written as an int but read as a long.

On msp430 an int is 2 bytes, while a long is 4 bytes. This means that 2
bytes are written to the padding, but 4 bytes are read from it: it has
only been partially initialised.

nano_malloc is the default malloc implementation for msp430.

This patch changes the cast from (int *) to (long *). The change to
nano_malloc has has been observed to fix a TI Energia project that
had been malfunctioning because malloc was returning invalid addresses.
The change to nano_memalign is based entirely on code inspection.

I've built and tested as follows:
  Configured (gcc+newlib) with: --target=msp430-elf --enable-languages=c
  gcc testsuite variations:
    msp430-sim/-mcpu=msp430
    msp430-sim/-mcpu=msp430x
    msp430-sim/-mcpu=msp430x/-mlarge/-mdata-region=either/-mcode-region=either
    msp430-sim/-mhwmult=none
    msp430-sim/-mhwmult=f5series
My testing has shown no regressions, however I don't know if the gcc
testsuite provides sufficient coverage for this patch?

I don't have write access, so if this patch is acceptable after review,
I would appreciate it if someone would commit it for me.

Thanks,

2017-01-XX  Joe Seymour  <joe.s@somniumtech.com>

	newlib/
	* libc/stdlib/nano-mallocr.c (nano_malloc): Fix incorrect cast.
	(nano_memalign): Likewise.

newlib/libc/stdlib/nano-mallocr.c

@@ -314,7 +314,7 @@ void * nano_malloc(RARG malloc_size_t s)
 
     if (offset)
     {
-        *(int *)((char *)r + offset) = -offset;
+        *(long *)((char *)r + offset) = -offset;
     }
 
     assert(align_ptr + size <= (char *)r + alloc_size);
@@ -587,7 +587,7 @@ void * nano_memalign(RARG size_t align, size_t s)
             /* Padding is used. Need to set a jump offset for aligned pointer
             * to get back to chunk head */
             assert(offset >= sizeof(int));
-            *(int *)((char *)chunk_p + offset) = -offset;
+            *(long *)((char *)chunk_p + offset) = -offset;
         }
     }