Fix crash in cpuid_riscv64.c

The crash is reproducible when building OpenBLAS without forcing a
target in a riscv64 container running on an X86_64 machine with an
older version of QEMU, e.g., 7.0.0, registered with binfmt_misc to
run riscv64 binaries.  With this setup, cat /proc/cpuinfo in the
container returns the cpu information for the host, which contains a
"model name" string, and we execute the buggy code.  The code in
question is searching in an uninitialised buffer for the ':' character
and doesn't check to see whether it was found or not.  This can result
in pmodel containing the pointer value 1 and a crash when pmodel is
defererenced.  The algorithm to detect the C910V CPU has not been
modified, merely fixed to prevent the crash.

A few additional checks for NULL pointers are added to improve the
robustness of the code and a whitespace error is corrected.

cpuid_riscv64.c

@@ -86,23 +86,29 @@ int detect(void){
   char *pmodel = NULL, *pisa = NULL;
 
   infile = fopen("/proc/cpuinfo", "r");
+  if (!infile)
+    return CPU_GENERIC;
   while (fgets(buffer, sizeof(buffer), infile)){
     if(!strncmp(buffer, "model name", 10)){
       strcpy(model_buffer, buffer);
-      pmodel = strchr(isa_buffer, ':') + 1;
+      pmodel = strchr(model_buffer, ':');
+      if (pmodel)
+        pmodel++;
     }
 
     if(!strncmp(buffer, "isa", 3)){
       strcpy(isa_buffer, buffer);
-      pisa = strchr(isa_buffer, '4') + 1;
+      pisa = strchr(isa_buffer, '4');
+      if (pisa)
+        pisa++;
     }
   }
 
   fclose(infile);
 
-  if (!pmodel)
+  if (!pmodel || !pisa)
    return(CPU_GENERIC);
-   
+
   if (strstr(pmodel, check_c910_str) && strchr(pisa, 'v'))
     return CPU_C910V;