assemble.c: avoid a panic after invalid instruction error

If an instruction triggers an error (nasm_nonfatal) it is legitimate
for calcsize() to return 0; in that case no further processing is
needed.

Signed-off-by: H. Peter Anvin (Intel) <[email protected]>

Author: H. Peter Anvin

asm/assemble.c

@@ -983,7 +983,11 @@ static int64_t assemble(insn *instruction)
             }
 
             data.inslen = calcsize(instruction, temp);
-            nasm_assert(data.inslen >= 0);
+
+            /* This can happen if the instruction generated an error */
+            if (data.inslen <= 0)
+                return 0;
+
             data.inslen = merge_resb(instruction, data.inslen);
 
             data.insoffs = 0;