Fix problems parsing RISCV architecture extenstions in the assembler.

* config/tc-riscv.c (riscv_set_arch): Handle the Q subset like
	all other subsets.
	Obviate use-after-free.

gas/ChangeLog

@@ -1,3 +1,9 @@
+2017-07-28  Andrew Waterman  <andrew@sifive.com>
+
+	* config/tc-riscv.c (riscv_set_arch): Handle the Q subset like
+	all other subsets.
+	Obviate use-after-free.
+
 2017-07-25  Nick Clifton  <nickc@redhat.com>
 
 	PR 21739

gas/config/tc-riscv.c

@@ -147,8 +147,8 @@ riscv_add_subset (const char *subset)
 static void
 riscv_set_arch (const char *s)
 {
-  const char *all_subsets = "imafdc";
-  const char *extension = NULL;
+  const char *all_subsets = "imafdqc";
+  char *extension = NULL;
   const char *p = s;
 
   riscv_clear_subsets();
@@ -173,7 +173,7 @@ riscv_set_arch (const char *s)
 
       case 'g':
 	p++;
-	for ( ; *all_subsets != 'c'; all_subsets++)
+	for ( ; *all_subsets != 'q'; all_subsets++)
 	  {
 	    const char subset[] = {*all_subsets, '\0'};
 	    riscv_add_subset (subset);
@@ -188,7 +188,8 @@ riscv_set_arch (const char *s)
     {
       if (*p == 'x')
 	{
-	  char *subset = xstrdup (p), *q = subset;
+	  char *subset = xstrdup (p);
+	  char *q = subset;
 
 	  while (*++q != '\0' && *q != '_')
 	    ;
@@ -200,7 +201,6 @@ riscv_set_arch (const char *s)
 	  extension = subset;
 	  riscv_add_subset (subset);
 	  p += strlen (subset);
-	  free (subset);
 	}
       else if (*p == '_')
 	p++;
@@ -211,15 +211,11 @@ riscv_set_arch (const char *s)
 	  all_subsets++;
 	  p++;
 	}
-      else if (*p == 'q')
-	{
-	  const char subset[] = {*p, 0};
-	  riscv_add_subset (subset);
-	  p++;
-	}
       else
 	as_fatal ("-march=%s: unsupported ISA subset `%c'", s, *p);
     }
+
+  free (extension);
 }
 
 /* Handle of the OPCODE hash table.  */