RISC-V: Fix build failure for -Werror=maybe-uninitialized

Commit 40f1a1a4 ("RISC-V: Output mapping symbols with ISA string.")
caused a build failure on GCC 12 as follows:

make[3]: Entering directory '$(builddir)/gas'
  CC       config/tc-riscv.o
In file included from $(srcdir)/gas/config/tc-riscv.c:23:
$(srcdir)/gas/as.h: In function ‘make_mapping_symbol’:
$(srcdir)/gas/as.h:123:15: error: ‘buff’ may be used uninitialized [-Werror=maybe-uninitialized]
  123 | #define xfree free
      |               ^~~~
$(srcdir)/gas/config/tc-riscv.c:487:9: note: ‘buff’ was declared here
  487 |   char *buff;
      |         ^~~~
cc1: all warnings being treated as errors
make[3]: *** [Makefile:1425: config/tc-riscv.o] Error 1

This is caused by a false positive of "maybe uninitialized" variable
detection (-Wmaybe-uninitialized).  To avoid this error, this commit
initializes the local variable buff to NULL first in all cases.

gas/ChangeLog:

	* config/tc-riscv.c (make_mapping_symbol): Initialize variable
	buff with NULL to avoid build failure caused by a GCC's false
	positive of maybe uninitialized variable detection.