tools/gf_poly.c: fix undefined allocation of 0 bytes

Due to man page of malloc the behaviour in case of allocation size of
0 bytes is undefined: "If size was equal to 0, either NULL or a
pointer suitable to be passed  to free() is returned"

Fix for clang scan-build report:

Unix API     Undefined allocation of 0 bytes (CERT MEM04-C; CWE-131)

210 poly = (gf_general_t *) malloc(sizeof(gf_general_t)*(n+1));

    9 Call to 'malloc' has an allocation size of 0 bytes

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

tools/gf_poly.c

@@ -52,6 +52,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <assert.h>
 
 char *BM = "Bad Method: ";
 
@@ -203,9 +204,14 @@ int main(int argc, char **argv)
       sprintf(string, "Argument '%s' not in proper format of power:coefficient\n", argv[i]);
       usage(string);
     }
-    if (power < 0) usage("Can't have negative powers\n");
-    if (power > n) n = power;
+    if (power < 0) {
+      usage("Can't have negative powers\n");
+    } else {
+      n = power;
+    }
   }
+  // in case the for-loop header fails
+  assert (n >= 0);
 
   poly = (gf_general_t *) malloc(sizeof(gf_general_t)*(n+1));
   for (i = 0; i <= n; i++) gf_general_set_zero(poly+i, w);